In my experience, it seems it's mostly them not wanting your amp to damage their cab. I once asked if I could bring my GK MB500 and was told no, for that very reason. The NYC music scene could not care less about the actual music. All they care about is how many people can you get in there to buy drinks and such. NYC is a really terrible music scene.
Sadly, not caring about the music has become the norm in a lot of the smaller Austin venues (bars and clubs).
