I don't know if this guy is still here, but it seems like common sense. Teachers are in the middle of their day, and like cliques of kids, they have their friends and habits... They're just doing what they usually do...not taking time to greet the sub, who they may never see again. I think if you approached a table, and asked if you could sit, they'd be friendly. I have done this...and also been the fortunate recipient of a few kind teachers' invitations.
I can WELL say why you shouldn't be teaching. You aren't qualified. Subs should do as they're told in the classroom. My teachers (thankfully) leave lesson plans, and I follow them to the t.