These booths are setup throughout the airport and anyone who wants to solicit anything may do so freely. Although they aren’t allowed to hand out fliers from anywhere except behind the booth, apparently they are permitted to yell across the terminal to everyone who walks by.
Not to mention they are the shadiest looking people alive, it is especially annoying when you have a long layover and have to hang out in the terminal for an extended period of time. I am not sure what the logic is behind having these booths hanging around an airport – just be aware of them if you happen to land San Francisco International Airport. Many people mistake them for information booths – those are setup in the center of the walking area, not near the entrances like the free speech booths fyi.