I think you give up your personal space the moment you board public transit. It peeves me when people put their bag next to them to prevent people from sitting down - especially as the bus fills up.

I admit I am one of those people you hate. There are several reasons why I do this:

- I prefer to keep the front seats available for the infirm, elderly, families, and those with poor mobility. It really quite angers me to see able-bodied persons take these seats.
- It is generally a rule of thumb - etiquette - to depart via the rear door. I choose any available seat near it regardless if the other is occupied.