It's a private bus, and they can make the rules.
Unfortunately, some people lose common courtesy when on the phone. I can pretty much block out two people talking, but only hearing one opine aloud is sometime annoying. It's fine while shopping, but if you're in line to check out, hang up the phone. And if you're at the library...did you know the Dallas Public Library says the law does not allow them to post a sign saying no cell phone use inside?