Treated badly? How is anyone supposed to know that you bought the second seat? Was there a "Reserved" sign on it? Not to mention, seats aren't assigned beforehand.

At the same time, I can relate. On my first chicken bus trip, in Nicaragua, I paid the cobrador (person who collects money on the bus) for two seats, since I had a lot of luggage. He didn't bother me afterwards, but I did get some displeased glances from fellow gringos. My brief moment as a fachento.