If the channel carrying a particular team's games, usually meaning a regional cable outlet rather than just old fashioned over-the-air TV, is available, or potentially available, in a particular area then that area gets blacked via mlb.tv in that region. The idea is to 'protect' that channel (several of which have been on the brink of insolvency lately) by not giving potential subscribers a work-around to paying for the service.
So, for example, a western Pennsylvanian fan can't get Pirates games via MLB.tv because he/she can get them through the Pirates channel. But if that same person is in an area that also has access to the Ohio sports channels then Reds & Guardians games might also be blocked.