Wow. I've never been asked to leave my dive knife behind in CZM, but I wouldn't call it a BFK, either.
Knives and gloves are not allowed in the marine park in CZM. That's why I carry my folding knife in my pocket (along with gloves in case I need them). The BFK is a foot long. Whether or not the dive op enforces it is up to them. They are the ones who get fined. Just sayin'
Cheers -