I don't take public transportation much so I don't have that problem, for the most part. But if I have the option, I don't sit right next to an occupied table in a restaurant if there are empty tables else where. Now if I am at a bar, a pub or restaurant bar, I usually strike up conversation with the people around me but I won't sit right next to someone if there are empty seats further down. I just feel weird if someone does that to me, so I don't do it to them. *shrugs* But I usually don't move, occasionally I will but not usually.