Unless someone doesn't fit onto one seat when they should paid for two, because it is rough on whoever gets the seat next to them.

To buy two seats just to have the space, is against the whole spirit of communal/public transport. Just because you can buy two tickets doesn't mean you should. If you don't like random people and coaches, check out Net Jets.

All that really happened is you had to explain the situation of you having two tickets. What did you expect, people to see an empty seat and ignore it when loads pax on standby?