I don't know about the expectation that a Standing O is requisite. I've only been to a few shows actually on Broadway, but see all the productions here in Kansas City. A year or so ago, a production of Kiss Me Kate came through, and I couldn't wait to get out. There was no Standing O, hardly any applause at all to justify thier curtain call. I'll only stand if the performance dictates that I get up.
Now, at the big Starlight Theather (outdoor theatre with 8000+ seats), many people head to their cars during curtain call or closing numbers. Prior to the show, the exec. producer emplores patrons to remain in their seats as a sign of courtesy, and theatre-goers appreciate his request. But for those who want to beat the traffic, it's a stampede. On those really hot nights, I can empathize, but when it's nice, let's stay and enjoy the cast.
What's that I smell in the air?
The American dream.