Because unlike our world, the Star Trek world actually respects people's privacy. Ever noticed how people just vanish from the ship and the computer never alerts anyone until someone asks for their location? When Trek was written, the idea of constantly monitoring and reporting on individuals was abhorrent. It's disgusting how willingly people just accept that now.
That's pretty much exactly how it seems to me. I guess I understand how American fans who were born after 9/11 and Facebook might have a different perspective, because privacy means something different now--but it's cognitive empathy, which means I understand their feelings, not the sympathetic empathy of someone who shares it.
Ironically, I learned these cognitive empathy skills from Captain Picard, and still consider TNG possibly the best way to expose young people to the skill. :-)
But like, they can still track you. And removing the badge that lets them track you is basically a crime. Also section 31 exists basically just to track and monitor people.
Section 31 were created as the bad guys! Genocidal maniacs who Sisko and crew fought against every step of the way.
And I don't use the phrase "genocidal maniacs" lightly, but they were literally xenocidal and Sloane was, as a spy, less of an Ian Fleming James Bond type and more of a John le Carré type—an actual maniac in the piece of human wreckage who's been turned violent and crazy by the stress of war.
(I really wish his end had come at Sisko's hands, and involved contrasting Sisko's actions in Pale Moonlight with Sloan and 31's degeneration in to xenophobic crimes of extermination, and how both shared the same origin but ended up in very different places.
I think the canon reason given for this and other "why didn't the ship's computer just stop them?" situations that it's a privacy violation to just go around scanning people without their permission.
Although they do seem to do a lot of "scanning for life-signs" so who knows?
Yeah, and I've never figured out the security feature that makes scanning for life-signs more effective when you sign a little song to the computer. But sometimes I guess it's just more urgent to know, little life signs, where are you?