I always like to think that after NEM, B-4 was taken by Commander Maddox back to Starfleet Science and taken apart, so as to better study and understand the Soong-type Android.
So Starfleet executed the slow android? Beautiful.
I prefer the term dissassembled
It's been ages since I saw the ep where Data was on trial, but if memory serves, he was granted sentient status and equal rights, not all androids. There would need to be another ruling on B-4.