Maybe I read it wrong, but here in the US I've always thought it was considerate to switch to an empty row if one frees up to give you both some more room.

Most people here in the US will actually put there bags in the empty seat until the bus starts to fill up and someone has to ask if they can sit there so they are forced to move the bag. LOL

I'm an introvert myself so I like that this is the social norm.