Funny, I checked out the reviews of "The Purge" at IMDb and saw that it had an overall rating of 5.5 which seemed kinda low to me. Then I checked out many of the negative reviews and there seemed to be a common theme, "The idea of a 12 crime purge is absurd" according to many of the negative reviews. The second biggest complaint was about the characters themselves doing dumb things, which is pretty common complaint with horror/suspense movies in general.
I guess I'm surprised at folks inability to suspend belief for 90 minutes or look outside the box at what could potentially happen if the stars were aligned correctly. For most movies I think you have to suspend belief to one extent or another... or you wouldn't have the premise of the movie, and consequently no movie.
The second complaint about dumb characters has merit– to a point. Again without a character doing something "dumb" or at least inconsistent to us, the premise of the movie wouldn't develop and we wouldn't have the movie. Many times you simply need one to get the other.
Bottom line I watch movies to be entertained... If I want 100% accuracy (or as close to it as I can get) I'll stick to documentaries.