If someone is IP hopping, that makes it problematic. If we go and ban each IP, we can track the offender a lot easier. Some of these people have this mistaken belief that we can't catch them if they just unplug their router or something.
Incidentally, said spammer is also getting himself a break for awhile. While we don't ban stupid, there is no reason excessive spamming posts need to be tolerated. When he can behave, he can come back.