You absolutely must watch this episode https://en.m.wikipedia.org/wiki/The_Measure_of_a_Man_(Star_Trek:_The_Next_Generation) the entire premise is that Picard provides a legal defense for the personhood of Data. The episode is literally a court case and it is one of the most well received episodes.

I think your slavery analogy holds. I don't think the Federation believes Data can be a slave because they don't believe he is sentient. But they do pick and choose how to apply their legal code to Data.