I can see both sides, but...if the seat isn't being used, the people that have the seats don't show up and you're not crowding someone, why get upset?
We had one game where only 4 of us were able to attend. We had 6 seats. We still fought to have enough room because the people to one side were notorious for bringing in extras. At another game, I was resting my foot on the seat in front of me (it was in a medical boot due to a foot fracture). The woman two seats over kept sliding closer until she was in the seat next to my foot. She kept giving me dirty looks. I thought she was going to say something and, if she did, was going to present the two tickets showing we paid for the seat she was in and the one my foot was resting on.