While "that would be stupid" doesn't reliably keep government officials (or companies) from doing that thing, not returning the phone promptly would be a really dumb move. They're not going to learn much from the device, and a phone is something that is incredibly annoying to lose for any amount of time.

If they keep it and it becomes known, people who e.g. took videos of incidents would become reluctant to come forward and share them, worried that the government might want to take the phone that recorded the video. They'd lose a lot of useful evidence in future cases, on top of the terrible PR it'd be.

Edit: The agency also relies a lot on the goodwill of the public. Investigations work a lot better when school teacher Bob, finding an aircraft piece in his back yard, contacts the NTSB and tells them to come get it, rather than deciding to quickly bring it inside and turn it into a coffee table later. And public perception can totally make the difference between "this might help them, let me call them immediately" and "screw those guys, it's mine now, will make a really nice coffee table".

I wouldn’t pretend to know what the NTSB considers potentially useful. I would expect them to prioritize anything that could remotely help an investigation over getting an iPhone quickly back to its owner.