If you bought two tickets but only one passenger showed up, the other passenger is a NO SHOW and the bus can assign their seat to someone else. Who told you that you could buy an extra seat and use it yourself?

Far too many people get on a bus and try to hog two seats for themselves, because it is more comfortable. The polite ones then get stuck with a seatmate while the rude ones get more space. This game is well known to frequent bus travelers.

I don't think you were treated badly. I'm not sure what you expected to happen differently.