There are several explanations for this. In general, if you arrive late at the gate, you're subject to being removed from the flight. They don't do this just because they're mean and want to punish people, but in order to accommodate standbys. If in your case there were four late-arriving passengers but only one passenger on the standby list, then I can easily see how only one of the late passengers (i.e. you) gets removed from the flight. They don't remove all 4 late passengers and prevent them from boarding just because they need 1 seat for a standby "just to be fair." Sometimes you get to be the unlucky one. In your case, there was an additional factor: you were in FC. If the standby passenger happened to be a displaced FC passenger, then guess which late passenger gets their seat taken?

Of course, this isn't to say that there aren't sometimes GA shenanigans, but for the most part, when I read stories like the ones in this thread, there are perfectly reasonable explanations other than GA malice that can explain the situation.