They don't break any laws or regulations currently in place.
Why should they be banned?
Because they have zero utility for self-defense or hunting or
even recreation.
Basically, the idea behind a bump stock is that you can take the recoil of a semi-automatic weapon to make it behave like it's fully automatic. While there's nothing specifically terroristic about an automatic weapon, the fact is that a bump stock makes your gun jump around like crazy, which makes it useless for basically everything except trying to mow down dense, large crowds of people.
People buy bump stocks as a novelty to use on firing ranges. They don't really serve any practical purpose to anyone who isn't trying to commit a mass-shooting.