One reason may be that AT&T and Google's answers are also now public, so Apple wanted to have it's best face forward.
http://www.scribd.com/doc/18983512/ATT-Response-to-FCC-iPhon...
http://www.scribd.com/doc/18983640/Google-Response-to-FCC
Personally, I suspect that Apple would prefer that AT&T take the blame for as much as possible.
From the tone of the letter, it sounds like Apple tries to shift blame onto AT&T where they can- For instance, Apple was responsible for the initial SlingPlayer rejection.
Apple's response says that they rejected the app because it violates AT&T's TOS, but doesn't say that AT&T requested the rejection.
Viewed in the context of the Google Voice rejection, this leads me to believe that Apple is happy to be able to point to the TOS as the reason for the rejection, rather than having to decide if they want to allow an (indirect) competitor to their iTunes TV store.