In reply to stp:
I think the rules came in to prevent people hogging the campsites all summer - there was a Yosemite scene in the 1960s and '70s where this was common. When I last visited (in the mid 1990s) you could extend the 7 days by moving between pitches, it was a bit like parking in a limited time parking space then simply moving your car to another space at the expiry time, it was a case of sidestepping the rules. The park authorities might have clamped down on this now.
Once outside the main holiday season I suspect that they aren't quite so strict about the time limits (but then again this is America and you must follow the rules) but I certainly wouldn't want my seven day limit to expire on Labour Day weekend (basically the same as our August bank holiday) for example.