I think this revulsion to sitting with others (ironically going to crowded thheme parks

) it the result of US being such a "car culture," where people go from house to car to work cubicle, and barely have to have contact with those outside of their bubble/circle, what-have-you...no one you don't perceive as "like you."
Also, all this FACEBOOK "friend" crap, is another way to distance yourself from others: PC friends who may be imaginary or real...(you certainly never have to sit with them!

)
I was born and bred in NYC, where we walk, and talk to a variety of characters on any given day. When people say NYers are rude, they are usually referring to transplants, not natives.
Most people near me, and those I've grown up with, love the OPPORTUNITY to meet new people, and maybe learn something, laugh a bit. We expect to sit with others.
Still, bad behavior is not acceptable, and I understand having an experience that makes you wary.