(author here) My experience has been that if you're up front with the police about what you're doing they won't beat your door down and haul you away. What usually happens is that the police look up the IP address they've identified as a problem and then check the ARIN database to see who owns it. ARIN is going to give them the name of the ISP not of the subscriber, so they're accustomed to then calling the ISP to subpoena the name of the customer. If you purchase service from a fiber provider then the name in ARIN's database will either be your provider or (if they SWIP the IPs to you) the name you give your ISP.

In the past what I've seen is that the police will ask you to identify who was using an IP and also to help them catch the person in the act if they do the thing again. So far I haven't seen them get too pushy if you can't do the first thing as long as you're willing to do the second.