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While that's true, my main argument was that Internet address systems are usually far less permanent and unique as they are often not hardware based but instead flexible logical systems, like your IP.
I imagine there are also laws against using hardware addresses to isolate personal information to a particular person, essentially spying on them, at least without a warrant.
Oh, but that's just subnetting. The ISP still knows what machine goes to what gateway, and what MAC was assigned what IP. Otherwise you wouldn't actually be getting those packets out and back. They'd be going to the next guy assigned your IP, or the last guy assigned your IP. Maybe the last guy who had your IP would be downloading you a whole passel of donkey porn.
No laws against the ISP telling this information to the government, no. There are certain laws telling them when they must, and they tend to cooperate. One hand washes the other, you know.
EDIT: Well, for clarity's sake...they know what IP address range they've assigned your location, what private networks you make on the other end of the router through which they communicate all traffic to and from your network are no nevermind to them.