[Symmetry]

IP/MAC spoofing is not intrinsically a fraudulent activity

No, it's not. But doing it to get around people blocking your computer is. Likewise firing a gun isn't illegal, but attempting to hit someone when you fire a gun is illegal even if you miss. Intent figures into guilt and inncense in most matters of criminal law.

Swartz clearly knew that his IP address and then MAC address were being blocked specifically. You could argue he might have thought that it was just traffic shaping, but that wouldn't explain the MAC block.

EDIT: Clarified by adding 'criminal' in front of law.