Consider the following scenario:
The round is scheduled to start at 6:30pm and normally I would expect my game to finish comfortable before 11pm when I need to leave to catch the last bus home at 11:05pm. There is no zero tolerance. Instead players have one hour to arrive before they are defaulted. Normally I start my opponent's clock at 6:30pm on the dot if he hasn't arrived so that I have no problem with the last bus.
At 6:25pm the arbiter announces that he has had news of an accident on the motorway which is delaying some of the players. Consequently he rules that we may not start a player's clock until 7pm if they are not in the club. The default time of 7:30 is unchanged.
I complain bitterly to the arbiter but he is unmoved and won't change his decision.
My opponent eventually arrives with the other late players at 7:05pm. I started his clock on the dot of 7:00pm. The game drags on and on. Eventually at 11pm I queen my last pawn against my opponent's bare king. I offer a draw but he refuses. I knock my king over in disgust and head for the door and the last bus.
The next week the arbiter tells me that knocking my king over constitutes resigning. He also tells me that if I had walked out without resigning I would have "lost on time" but been awarded a draw because my opponent didn't have mating material.
This is perfectly legitimate. I have resigned against a bare king and this is legal. The only sanction I am likely to suffer is for not shaking hands after the game.
EDIT: This subject actually came up at the recent FIDE meeting in early September! Alex Holowczak (amongst others) was representing England at these meetings and his report appears here - http://www.englishchess.org.uk/wp-content/uploads/2015/10/C188.8.131.52-FIDE-Delegate-report.pdf .
Note, in particular this excerpt from Alex's report from the meeting of the Rules Commission:
A proposal noted that you could run out of time but not lose if your
opponent had a bare King, and asked why you should lose if you resign
but your opponent had a bare King. I commented that this situation was
so unlikely/unreasonable that it shouldn’t be legislated for.