Nothing irritates me more than when hotels selectively apply their rules. I've stayed in many hotels where the "Do Not Disturb" is blatantly ignored. Of course, if you accidently leave the sign out, the Housekeeping Staff will quickly point to this as the reason why the room wasn't cleaned.

I'm a former Assistant Front Desk Manager of a chain hotel. I also worked as a Night Manager at a Preferred Hotel. In my opinion, it all just comes down to training. I can assure you that the only time we ever ignored a "Do Not Disturb" sign in either of those hotels was if it was after check-out time and there was no answer to repeated phone calls to see if a guest left.

Unfortunately, the training of high quality hotel employees seems to be few and far between. I was pretty fanatical about it. I even insisted that my Front Desk employees hand guests a pen to sign the registration card instead of putting it on the desk. (Way back when people had to sign for things!)