To me it's more like the sign saying "No guns in the bank". It's not going to stop someone who intends on doing harm, but that's not it's real purpose. If it was normal to be able to openly carry a gun into the bank, that makes the security guards job much harder.
If someone is mounting weapons on a drone, a flight restriction isn’t going to stop them.
It’s like putting a sign on a bank that says no bank robberies. Guess who isn’t going to obey the sign? People who rob banks.
The FAA rule isn’t there to stop bad actors. It’s there to prevent bad actors from being recorded.