My recent experiences:
One place the waitress presumed I wanted to sit at the bar (the place looked pretty full, but after when two guests came in together and she directed them to the bar and they asked if they couldn't sit elsewhere she sat there is plenty of space and led them in). Admittedly she was on the phone so couldn't really talk much other than motion with hands.
Another place there was no bar, so I sat at a table for 4 all by myself. I was wondering if the waiter was going to try to move me because a party of three came in after with nowhere to sit. But no, I guess in America they don't move you just because you don't take up all the seats.