Justice can be turned off but only manually by a GM so it's not going to happen during one of the auto-invasions that players initiate with the wardens. There's too much potential for abuse if players have the ability to turn off justice themselves.
In general, a disturbing the peace charge comes from any spell that CAN affect the area, even if you target only invasion creatures. It's not that crazy when you think about it - most towns would probably not want explosions going off on the streets, even if it doesn't hit anyone.