What's the point?

So, first off, no I'm not staying at the Ritz. I just grabbed this picture off of the net.

I am staying at a really nice hotel though. The Radisson in Merrelville, IN. My point is I often wonder what is the point of the Do Not Disturb sign. I'm in my room working on the PC and sure as crap with that big ass DO NOT DISTURB sign on my door housekeeping still bangs on the door. In most places if they don't come knocking on the door if you have your DO NOT DISTURB sign up then they will call you on the phone and wake you up that way.

ADDENDUM:

So, I went out for a few hours and I get back to my room and sure enough housekeeping is my room cleaning. ARGHH!! I hate that, see, I'm such a slob that I feel like I have to clean my room before housekeeping can come in and clean my room.