Why would it be disabled?

Lolzy

Why would BRML be different from other materials?

brml flood fill works fine in 40.2

I still can't flood fill photons either D:

Actually, that makes sense, I think. From my point of view, every existing element should be selectable in the menu and usable in all standard ways like flood fill etc.
Exceptions are clearly spark, photons etc, but a sandbox is only a sandbox if limitations are kept at a minimum. That would include the availability of smoke in the menu.