It seems very straightforward to me.
- If you train your dog and use him as a defensive tool, you have to be able to control him. So if someone attacks you, you set the dog on him, and then he retreats, you have to be able to call the dog off.
- If your dog is simply protective, that's how dogs work. It's your responsibility to make sure he isn't a loose cannon, but if you and the dog are minding your own business when you're attacked, and the dog jumps to your defense, that's not you using a "weapon" on someone.
The thug has created that situation by attacking you. He might change his mind halfway through when the dog stands up to him, and like a reasonable human being you'd try to call him off, but he can't complain if he gets torn up.
It's little different from if he tried to burn your house down, had a change of heart halfway through, but got trapped and died. Unfortunate, but he created an uncontrollable situation.