To gain legitimacy by establishing a legal precedent. This one is pure speculation. Let's say the government would like greater control over your electronic communications. That is fine, as long as they know where you are, and which server you use. The way the Internet was originally set up, if one really wanted to remain anonymous and untraceable, one could get lost in the sauce.
Spammers are a paltry minority compared with legitimate users of distributed networks.
Whether from another account at work, while traveling, from an Internet cafe, friend's place, totalitarian regime country, hidden from the nosy boss, etc., users naturally want to be able to access and make full use of their accounts. They may also want, for a number of private reasons, to be able to send or receive communications on the Internet similar to using a public phone, rather than dial in. Remote use doesn't happen through the ISP's local dial-in network, and such communications are tough to control and monitor. Again, pure speculation. But it may, hypotetically of course, be a reason why the Department of Justice failed so far to act against MAPS blackholing with impunity selected parts of the Net at will.