This contemptible approach turns your rightful marketing costs into negative externalities for everyone else, filling a formerly efficient market with nonexistent bullshit. I'd love to see the FTC nail anyone guilty of this, though they probably can't afford to keep up. Maybe we need a blacklist.
EDIT: some of the examples involve actually providing the service you're advertising, albeit in an unscalable way, and that's perfectly legitimate. Just don't promise something you aren't at all ready to deliver.
What about Dropbox's method? There was no lead-gen site that tried to imply for 99% of it that the product already existed; instead, there was just a video containing synthetic "this is how we imagine this working" flows, and a request for people to sign up if they were interested in seeing that vision realized.
If "this isn't available" was clear up front, it sounds like a volunteer market research panel, which is ethical iff requested openly (though I may not have been inclined to sit through it unless I also needed ideas).
EDIT: some of the examples involve actually providing the service you're advertising, albeit in an unscalable way, and that's perfectly legitimate. Just don't promise something you aren't at all ready to deliver.