It's a necessary blindspot we have to allow for, in order to have an otherwise good episode. Without glossing over that, we can't even have this story, because there's no cause for any kind of legal case.Why, in Measure Of A Man, did Picard not use Datas decision to join Starfleet as a reason why he wasn't property? The argument about the computer refusing a refit is moot as that was developed FOR Starfleet, probably by Starfleet. Data was not and he chose to enter the academy of his own free will.
Data's enlistment in itself is a logical recognition of the will of a free being. So, if you're looking for an explanation as to why it's not addressed, there isn't one, but if you want to engage in the exercise of trying to devise an in-universe explanation, then...
There's really only 2 options. #1, when you dig into the fine print, maybe joining Starfleet, on it's own, doesn't explicitly carry with it a recognition of personhood for anyone, which really seems like a very unlikely scenario, given their progressiveness.
Or #2, the rather contentious enlistment of Data himself brought with it some unspoken caveats to the general enlistment standards. That his enlistment was objected to officially by only Maddox, doesn't mean he was the only one with reservations. He was just the only one publicly on record about it, and others were also unwilling to grant Data the same rights as other recruits have, which somehow shut down Picard's claim that as a Starfleet officer he has certain universal rights, because they'd made special provisions for him.
So, we can guess that maybe Picard dug through Data's initial enlistment records & realized they had done things a little different with him then, than with others, which made the argument not have any use. It's a thin line of reasoning, but it's something.