Every couple weekends someone in my neighborhood has a big house party with a live band in the back yard. They are a few streets over, but I can hear clearly the music from my yard. I can even hear muffled low frequency content from inside my house.
No one complains. I assume because the band is not shouting 4 letter words and the loud music ends at a reasonable hour.
The live sound system for the party has more range than a part 15 complaint FM transmitter. What the band is saying is probably heard by more people. With Part 15 FM, someone needs to have their radio on the correct channel when they are in range of the transmitter. For the band to be heard, someone just needs to be in the general area with the car windows open or out for a walk.
So if the goal of a part 15 FM station is to spread political opinions, it may be easier to find out just how loud you can be and put a speaker at the edge of your property, while obeying all disturbing the peace, noise, and quiet hour laws. That is probably more defendable in court by the first amendment than any arguments that pirate radio is protected by freedom of speech.
An added advantage (I think, consult with a lawyer to be sure) of sending out content as audio waves rather than radio waves is who has jurisdiction to respond to complaints. If I broadcast with 251uv/m@3m on 88-108MHz, the FCC can enter my house without a warrant and seize all my transmitters, even if they have nothing to do with on 88-108MHz. If I have a speaker in my yard blasting my political opinions the police have very strict rules they should follow when they respond.
On the other hand, playing copyrighted music from a speaker in a public place is a public performance of a copyrighted work so that is a whole other set of issues.
This is fun to think about. What happens if two or more acoustic broadcasters are on at the same time? At least with radio we have a way to separate the signals.
In my community you can be cited for violation of a noise ordinance if your sounds are "objectionable at a distance of fifty feet to a person of normal sensibility." That's a lot more vague and subjective than 251 uV/m.
Neil