Assuming Comcast has a bandwidth cap, by spoofing real MAC addresses he's basically stealing other users' bandwidth.
But then again, people steal wifi all the time.
But then again, people steal wifi all the time.
If your modem's external MAC isn't in Comcast's (global!) subscriber table, they give your modem a "walledgarden" default configuration file that causes it to reset frequently, and a poisoned DHCP DNS that directs all queries to their "Subscribe Now!" webserver.
Some of the European ISPs have countermeasures that rotate the names of the configs frequently, but Comcast doesn't do that -- if you've modified your modem, all you have to do is have your modem ask for a different config file (the names of which it will sniff from your neighbors), and set a different DNS server on the modem's immediate clients.