Alot of people have no clue about this rule but certain states use it. Here is a little information on it.
Dillon's Rule is derived from written decision by Judge John F. Dillon of Iowa in 1868. It is a cornerstone of American municipal law. It maintains that a political subdivision of a state is connected to the state as a child is connected to a parent. Dillon's Rule is used in interpreting state law when there is a question of whether or not a local government has a certain power. Dillon's Rule narrowly defines the power of local governments.

I learned about this rule when someone's mutt decided to rip into a little girl. It wasn't an APBT but they decided to run with it and try to ban the breed here, fortunately Virginia is a Dillons Rule state and being a Dillons Rule state, they cannot ban a certain breed of dog in the state, they would have to ban all canine breeds in the state. Look it up for your state and it may help to fight BSL in your area.