And if the problem pops up for enough people, I think it is indeed a cop out to expect just the DM to take care of it.
Let's take for example something more concrete: Scry/buff/teleport.
Used individually they're ok. They fill certain necessities. But used together, they are breaking that style of play. And if enough groups put 2 and 2 together, then you have a lot of groups breaking play style.
Should the DM be expected to put his foot down, or should something be expected to be done with the rules?
The impression I get around here is that the DM is expected to address the problem, generally by going through a long, lengthy process of countermeasures to counter the PCs tactics, so that every single threat is hidden behind layer after layer of protections just so the players can't break the game. And if the DM doesn't do this, or does this and is frustrated by it, well then he's just a bad DM.
And scry/buff/teleport is not only bad DMing, it is even worse DMing.
This one is really simple:
'Hey guys, this is a really simple plan you have, but it will short out my entire adventure and mean the hours of work I put into this will be gone and I will have to ad-lib the entire night. I do not find that to be fun. I will let you guys do it this time, but after this, intelligent bad guys will know about this and have a counter.'
Too many people here look at it as black and white. Either it must be banned always or it will always break the game. Guess what, you can have grey. Dont want the players skipping your chasm dungeon? Make it a choice of fight the dragons and get no treasure, or you can climb the chasm and find my adventure. Players want to teleport into the throne room of the high priest/avatar/god-king of the circle of archmages? Wow, you just teleported into their anti-magic dungeons stark naked (after you warned them). Players want to teleport into the throne room of the baron of podunk-ville? One satisfying session of players putting the 'why are you hitting yourself' on said baron.
Another thing that seems to be missing is that in a game with 5 PCs, there are SIX people playing D&D. The DM isnt an observer, he is the guy with the most vested in the game. The DM is there to have fun too. He may have fun differently, and have different role to play in making the game go, but he is still there to have fun. When the players, wether deliberately or accidentally blow that out of the water, the DM can ask that something be changed, just like the players can too.
There is no perfect rules set. All games can be broken, and (nearly) all can be alot of fun. Depends on the players. One of the great strengths of RPGs instead of computer games is the ability to change the rules on the fly to take gaps in the rules system into account, or to change the game to taste. Do it. Dont be an analog console.
There is no such thing as a DM/player arms race. The DM always wins. May not be fun, but the DM can always kill the players, if for no other reason than he controls the game. 4 minions every 2 minutes will eventually kill the players. It is only a matter of rolling enough 20s. This dosent mean that the DM should or shouldnt kill the players, just a look at how RPGs are set up. On the other hand, DMs need to remember that they only have one brain, and the players usually have more. They will eventually be able to outhink anything you do, if for no other reason that they have gotten used to your tricks. They will eventually find the holes in the rules that you missed, if for no other reason than they have more eyes to read with. So when they come up with something nasty, steal it and use it against them. If they really hate it, let them come up with a counter, then steal that too. Then point out that the guys who are using their ideas against them are, in fact, just as smart and skilled as they are, if not more so.
So, to address the OP: Calling bad DMing can be a cop-out, but it isnt always a cop-out. When there is a trivial solution to a problem, yet a DM will not use it, and that causes players to not have fun, then the DM is not performing well.