Well, for starters, let's say you've got a robbery ring of people snatching iPhones in public places. Police track them, raid their meeting point, and proclaim this glorious bust with 67 arrests related to the nefarious criminal snatcher ring. What happens to the phones present onsite during the raid? Seized.
So they now have 300 seized smartphones, acquired legally. What do you think they do with them? Open 'em up, check the data, put it on the Fusion, of course! I mean, it's intelligence on the kinds of data criminals keep with them, so it's fair game, right? (Hint: Nope. But some police officers really do think this way.)
What's more... maybe some of these phones were already "cleaned out" by the snatcher ring, and maybe some weren't. Time for a subpoena to the phone carriers to know which phones belonged to whom! And, well, while we're at it, it might be useful to know if any of these phones actually belonged to some of the snatchers themselves, maybe it'll lead us to their contacts and to more of their affiliates. This is still criminal activity intelligence, right? So let's also get all the phone records and contacts and calling history for all the previous numbers and owners of all of these phones.
See how quickly this stuff escalates? And how easy it is for any officer or investigator to get carried away and justify it to anyone who asks?
(Note: I'm not saying this happened. It's a thought experiment.)