In all fairness, if I was standing on that bus and I saw your empty seat I would not be happy to be told that you bought the seat and want it empty. I understand that you had the right to the seat, but unless you're a person of size, have baggage or have a good reason for buying the second seat, I would just let someone sit there. Call it a good Samaritan rule or whatever you want, the bus ride is not a short ride, and its unpleasant to stand on a crowded bus for hours.