The group that owns the Red Sox also owns NESN. Similar for the Bruins. MLB can determine any national coverage for the Sox and all other teams, but because of ownership rights, NESN has to be compensated. So while NESN doesn't carry some games, they are financially compensated. There are a lot more details to this and someone else may be able to provide them, but NESN does OK even though they don't carry some games. Gives the NESN crew a break, too.