It's a movie "theater", applause and audience participation are a big part of traditional plays, movies not so much but it's still a part of viewing something with a large group. It's not an opera or something so I really can't understand this widely expressed sentiment that clapping is a terrible faux pas. I don't really do it myself but I've seen it done and it seemed appropriate, mostly when something is said that targets a specific group, and that group applauds. Got a good example; While seeing The Mummy in a theater I happened to be with a group of librarians, early in the move Rachel Wiess is drunk and trying to act proud and tough in front of Brendan Frasier, leading her to proudly exclaim "I... am a librarian!" drawing some applause and shouts of approval from the group I was with.
Sure it can be disruptive, and just constantly talking at full volume is very rude to others, but I'm just not feeling the hate, people kicking my seat draws my ire but not an occasional hoopla. I've never done much more than laugh during a comedy, or sometimes during an action movie, but i see the appeal.
I suppose it is a more American thing, never got to see a movie while I was in England, ya'll make it sound like a rather stuffy affair though. Heaven forbid someone went in a little loaded and couldn't contain themselves, probly get a very polite lynching afterwards.