cable and DT sub-channels

[Kevin Vahey]

Here is one for Scott

FOX and MLB have come up with a plan that next season every FOX
affiliate can carry every game offered by the network in their
exclusive Saturday window.

For example the Red Sox could be on 25.1, Yankees on 25.2, Cubs on 25.3 etc.

Question I have is do cable companies have to carry the sub-channels?
I know Comcast carries all the WGBH channels but is that mandatory or
voluntary.