My guess, for what it's worth, is the robbers (may they rot in a pile of maggot shit in Purgatory) is they were after the money and took the pipes because it was, after all, a pipe convention, so their little minds realized they must be worth something. But criminals are a lower breed of person and not as bright as the average individual and certainly not pipe aficionados. They just figured the pipes were frosting on the cake.
Herein lies the way the police with catch these creeps. Sooner or later they are going to try to unload the pipes. Where? Pipe stores or pawn shops, which will probably be alerted about the theft and to be on the lookout for the pipes. Problem is, they need to wait a couple of months until the heat cools before selling their loot. But the moment they try, WHAM!