A pilot can indeed have a passenger removed from the plane.

That said, this was handled very poorly by UA IMHO. So the captain went by some other pax's word you were "suspicious" because you were on a cell phone?! If that's "suspicious" then nearly everyone who's ever flown should be removed from a flight. Sounds like the GS paid-F pax wanted your seat and tried some despicable hardball to try to get it; they should IMHO lose their GS credentials over that, and even be banned from future flights on UA if that is indeed what happened.

I'm curious, but obviously this is a nosy question (and thus don't answer if you're uncomfortable doing so): are you of an non-Caucasian race? US is infamous for overreacting to pax-reported "suspicious behavior" by non-white passengers (recently diverting a plane over an orthodox Jew's prayers!), but I hadn't heard much along those lines with regards to UA--but there's always a first time.

Also, the TSA wasn't exactly making you or anyone safer with their behavior, job or not, but that's fodder for another forum.