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.
