It would also be unauthorized access to a computer network, which falls under terrorism statutes (which could mean a free paid vacation to the south coast of Cuba until they find your papers [which flew out of the plane over the ocean]).
Just changing the MAC address of a device itself is still a federal crime, regardless of why you do it. (Routers are still made with the ability to spoof MAC addresses, but that's a holdover from the days that the ISP would lock the account to one MAC address and, until you could get them and have them change the MAC address on your account to the one on your new router, you'd spoof the address of the old router. If two devices with the same MAC address ever appear on the same network (and the internet is a network), there's going to be a problem.