In my uni in Thailand, this would also be useful for kicking out users who suck up the bandwidth with their downloads. They screw things for the rest of us.
Peoples concepts of illegality are classified in psychology. Don't because they say so is one step up from I can do it because I have the power. A real healthy approach would be understanding the functions a law serves, and following the law based on our perception of the function that the law is serving.
So you're mucking around with ARP tables etc. Maybe you can explain how you can be so sure it doesn't mess up a router?