I thought I would update. Based on the responses here, I have simply said "excuse me" and moved toward open seats and sat down. Still, some people seem, I don't know, astonished that I would have the nerve to do that, even though others then followed suit, including the standing people who seem astonished that I say "excuse me" and move past them to a seat they previously made no move toward. I think it really just comes down to personality. I am not going to stand if I can sit on the bus, and I think some people need someone else to show them it's okay to sit in open seats. Baffling to me!