Which licensing terms? That has nothing to do with each other. This icon set is foss, that means the sources are free and you can use the icons as you wish, as long as not otherwise encumbered. For the brand icons the source code is free, but if you use them in a way that conflicts with the trademark holder rights they can sue (depending on your jurisdiction).
That's why patents are such a big danger to FOSS software, because that's an equivalent situation.
You have fallen into the confusing intersection between trademark and copyright.
Files created by Facebook, depicting the trademarked Facebook logo, are copyrighted by Facebook. Files created by Feather, depicting the trademarked Facebook logo, are copyrighted by Feather. If the latter is freely licensed, with no restrictions on use, you may use that image in whatever manner you please. If you use it in a manner that generates brand confusion with Facebook's business, Facebook may, as a trademark enforcement action, order you to stop using the image in that way. They cannot make additional copyright claims.
It is likely they will anyway, under the assumption that no one but Facebook would create files from scratch for corporate logo images, when the company itself distributes gratis images of its logo to anyone who wants to put one on their website. But if they didn't make it, they can't impose prior restraint on what you do with it via licensing terms.
Facebook may impose copyright licensing terms on its own images that support its defense of its trademarks, but those terms have nothing to do with the image by Feather, or any other clean-room implementation of the Facebook logo.
What about the derivative works right? Although not a literal copy, one could argue that making one's own similar version of the logo could constitute a derivative work that could be construed as infringing.
In this example, the copyright interest is in the digital file, not the subject matter of the image. As long as you do not use Facebook's own file, such as by wrapping a PNG downloaded from Facebook in an SVG, there is nothing to derive.
But if you did do that, there would also be nothing transformative either, so your new image file would not be eligible for protection.
If it seems stupid and counterintuitive to separate the property interest in the file from the property interest in the image described by the file, that's because it is. But some people wanted to be able to have copyrighted digital image files based on artworks that have been in the public domain for centuries, so here we are.
> In this example, the copyright interest is in the digital file, not the subject matter of the image.
That's not my understanding of what the derivative works protection covers. What else is to prevent me from making a new Star Wars film using Lucasfilm's (now Disney's) characters?
The Star Wars proper nouns are trademarked. Characters, vehicles, planets, etc. That is all.
You can make a Star Wars styled film that makes no mention of any existing Star Wars trademarks (an intellectual property minefield) and Disney couldn't touch you (legally). They could still smother you with their massive business negotiating leverage, by threatening to blackball anyone that screens your movie.
Since I can't seem to convince you, here's a definition of "Derivative Work" straight from 17 U.S.C. 101 (https://www.law.cornell.edu/uscode/text/17/101) (emphasis mine). (FWIW, I am a licensed attorney who specialized in IP law while in law school; this is not legal advice.)
"A “derivative work” is a work _based upon one or more preexisting works_, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, _art reproduction_, abridgment, condensation, or _any other form in which a work may be recast, transformed, or adapted_. ..."
As applied to the company logo in question, the language is pretty well on point. I'd tread with caution unless the company has made it clear they approve of the work.
I don't consider the Facebook logo itself to embody a level of unique creative expression sufficient to qualify as an artwork. Obviously, if contested, that decision could be overruled by a judge, but I like my chances there.
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| .-| Probably does not
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The bar for qualification as an artistic expression is extremely low. See _Feist Publications, Inc., v. Rural Telephone Service Co_, 499 U.S. 340 ("To be sure, the requisite level of creativity is extremely low; even a slight amount will suffice. The vast majority of works make the grade quite easily, as they possess some creative spark, "no matter how crude, humble or obvious" it might be.")
You'd be better off arguing some other angle, but please study law first so as not to waste anyone's time.
...as though only the opinions of lawyers count when it comes to the laws that govern everybody. This is why right to trial by petit jury and indictment by grand jury are written into the federal constitution and most state constitutions. The purpose of studying law is so that justice may exist in a society that has abandoned the use of force as the first means of resolving disputes. It is not to shut down a debate via appeal to authority.
What's wasting everyone's time is the rich and powerful trying to mine out and own every last corner of culture. I'll be damned if I will allow Facebook to own every blue square with a lowercase 'f' in it, rather than just the one they use to mark their service. You may feel otherwise, but until this matter actually goes to trial, that's just, like, your opinion, man.
I don't think that licensing terms is the right term here. But IANAL. I'd call it licensing terms if you are using the license of the trademark holder to do something with the icon, like marking a product of your own. That's not happening here. Feather is distributing icons of brands (which is legal) or icons that resemble those icons of brands (still legal) to people that may use them to depict those brands (for which one does not require a license).
Oh, seems like I'm out of my depth there :/ I do understand that copyright might apply, but not how it could make such logos problematic (in US-law, right?). In which way can such a logo be infringing derivative work?
That's why patents are such a big danger to FOSS software, because that's an equivalent situation.