I think at times DM's want (and I have played under a DM who wanted) their players to be babes in the woods when it comes to monsters. From a players point of view this is both frustrating and unrealistic. In every society I know of there have been tales told of heroes defeating whatever sort of monsters that society believes in. I would expect in a society where monsters are real this is even more true, heck there is a class based on the telling of tales.
Take for example the real world. A large percentage of people who are familiar with European folklore/tales if faced with a creature who has a serpentine body, naked female torso, and snakes for hair would know that you shouldn't look at it in the face, and track it's movements using a mirror of some sort.
Both as a DM and as a player I have always assumed that very basic knowledge especially about creatures that are either common or very iconic is well known by everyone. Compound this with the fact that a lot of adventurers stated goal is to become heroes and slay evil one can expect that they spent the time as youths listening to the bards tell tales of fighting monsters.
Back to the original complaint about imps. I agree that knowing the exact type of DR versus the different elemental types is metagamey and as a DM I would talk to the players later about that. However recognizing them as imps and as such are minor denizens of hell does not seem out of line. Nor would the extrapolation that well since they are from hell and hell is full of fire, therefore they probably are immune to fire. In the games I have run I use examples like this when talking to players about metagaming. Often it can lead to fun roleplaying and interesting situations when players use logic without enough facts and come to incorrect conclusions.