Wouldn't distributing that file be considered copyright infringement? Just because you can legally access it there doesn't mean you have the right to redistribute. Sure you could direct your students there, but you can't do the copying yourself if I am not mistaken. Seems like picking up 20-30 copies off amazon would be easier.

The intended use in important. If a teacher distributes the speech in order to teach a lesson about it, it is very different than if a business man sell a book of the speech. Furthermore, the negative publicity from suing a teacher, using Martin Luther King Jr's speech to teach about civil rights should make any teacher completely safe from lawsuits.

Possible publicity is a pretty weak protection compared to actual legality, and I can't imagine a regular teacher (I'm excluding highly idealistic teachers, but those are few and far strewn) would risk their job illegally distributing those texts/videos, if there is even a quanta of possibility that that happens.

Teachers aren't the idealized constructs they often use to teach.