Since when was it uncommon for someone allegedly involved (directly or otherwise) to be fingerprinted? So they made someone do it to a phone instead of an ink pad this time. What's the task difference here?
Here's the thing.
Say you're wanted for organized crime or terrorism charges. The cops get enough evidence to get an arrest warrant for you, and a search warrant for your properties.
When they arrest you, you get fingerprinted. During your arrest, per the terms of their search warrant, they confiscate every electronic device in your house.
You weren't alone when they arrested you though. Your live in girlfriend Tina was there, as was your buddy Mike, and a friend of his, Chico.
During your arrest, those three will most likely be searched for weapons, cuffed, have their names/ids ran, but this is not being arrested. This is temporary detainment pursuant to the execution of a search/arrest warrant. This won't show up on their criminal record. They won't be fingerprinted or interrogated. Once its determined that they do not have any weapons on them, or anything illegal found while searching for weapons, and don't have any arrest warrants, they will be let go. They'll let Tina grab her purse, after checking it for weapons, just like they'd let mike and chico grab their wallets had they left them laying somewhere, and things like clothing, jackets, shoes, etc. But nothing else.
The problem is, Tinas nice new iPhone 6 is sitting on the kitchen counter charging. It wasn't in her pocket or purse when they came in, so its confiscated pursuant to the search order.
Now, the police have your fingerprints, as you've been arrested, booked, and charged with a crime. The problem is, your fingerprints dont unlock Tinas phone. They wont just turn the phone over to Tina. It might have incriminating evidence on it. But Tina was never arrested or booked, so they do not have her fingerprints on file, so can not try to fool the phone with a copy of her fingerprints.
That's the difference here.