I don't see what firearms have to do with Lego pirate cannons.
I've heard that there is a US law governing the minimum size of toy projectiles (to prevent choking).
Since firearms are not toys, these rules do not apply.
One would wonder though if BB guns are considered toys or sporting goods. I'd imagine that a BB gun or a pellet gun is probably not considered a toy.
The foam pieces seem to be bigger - though I have seen toy guns that shoot small plastic projectiles in the cheapo toy aisle at the grocery store.
As for water, I wouldn't consider it to be a solid that someone can get caught in their throat - nor can it put an eye out.
Probably just a legal decision on the part of TLC.