I failed my Will save to resist replying to this thread.
I'm conflicted on this issue.
On the one hand, whining players annoy me; most of the time they'd be better off constructing a real plan of action, rather than relying on quoted/memorized DMG/MM rules to get the upper hand over a foe. And this situation is no different: if they'd listened to the legends their characters heard, instead of relying on their encyclopedic (player-) knowledge of things their characters have no notion of, they would be better off.
On the other hand, players also need to know the ground rules. I'm not saying Doc should have sat down and explained every little detail about how DR will work, just that the players should understand that Doc is running a game in which unfair character knowledge will not be tolerated. I think that sort of frank discussion would end up causing less problems in the end.
Y'know, in the last game session I ran, the PCs encountered a Roper. They were about the get their clocks cleaned, but the party wizard shouted, "Spells are no use! Fire! We need fire!" Soon the roper had surrendered, and the parleying/interrogation began. Unfair knowledge? In most cases, yes...but when this player first got Polymorph Self (PC was created as a back-up 10th lvl character), I made him write up a list of creatures his character had encountered in the past. He wrote it up as a journal, and included was the encounter he'd had with a roper in the Underdark years ago. The player felt bad for using unfair knowledge, but as DM I remembered the journal. It was all good. We talked things out, and came to an agreement about how player/PC knowledge would work.
I think the Doc has a right to do whatever he wants, but "fooling" the players in order to "fool" the characters will not only lead to RL conflict, it also smacks of an inability to draw a line between the two knowledge bases that fueled this whole situation!
Use of meta-knowledge is the "rule" Doc and his players should be discussing...not DR.