Okay, I've actually been swayed.
My position is now "A DM can't cheat
at D&D."
However, that is far different from "A DM can't cheat."
There are (at least) three fine lines to understand the difference between those two statements.
1.
Gaming group limits. This category is a separate set of rules from the D&D rules, though it may impinge upon, expand, or directly change the D&D rules as well. This includes a DM who agrees to never fudge and then fudges, a DM who agrees to use standard monster hp out of the box and then changes them, to run a module as written and then changing it, etc. The DM has not cheated
at D&D by doing these things, but he has cheated at gaming group limits.
2.
Organized/tournament play. This category is similar to #1, but different enough to deserve its own category. A DM playing as part of the RPGA, for example, is agreeing to play under their rules and to run things with only so many changes as are allowed (this is to balance the challenge/experience to be somewhat uniform amongst all players of the modules). If the DM oversteps his proscribed power limits, he has cheated not
at d&d, but he has cheated the organized rules.
3.
Standard play. A more complete quote from the section that Water Bob pulled his quote from is as follows:
Do you cheat? The answer: The DM really can’t cheat. You’re the umpire, and what you say goes. As such, it’s certainly within your rights to sway things one way or another to keep people happy or keep things running smoothly. It’s no fun losing a longterm character who gets run over by a cart. A good rule of thumb is that a character shouldn’t die in a trivial way because of some fluke of the dice unless he or she was doing something really stupid at the time.
However, you might not think it’s right or even fun unless you obey the same rules the players do. Sometimes the PCs get lucky and kill an NPC you had planned to have around for a long time. By the same token, sometimes things go against the PCs, and disaster may befall them. Both the DM and the players take the bad with the good. That’s a perfectly acceptable way to play, and if there’s a default method of DMing, that’s it.
Please note the last sentence. "That’s a perfectly acceptable way to play, and if there’s a default method of DMing, that’s it." The default method of DMing is for the DM to obey the same rules the players do. So while it is within the rules of D&D for a dm to occasionally fudge (occasionally is mentioned in that section on DM cheating as well), it is not the default method.
So, I'll agree that a dm who is unfair to players, makes things up on the fly, has a DMpc who the rules do not apply to, brings in Deus Ex Machina, etc is not cheating. Many of these are, or can be, instances of really crappy dming, but are not cheating.