HAM liceneses are fed regulated by the FCC and no one on any local level has any idea who may or may not have a license and/or equipment. This is probably their only way of knowing.
That's my guess.
It's technically true that the locals may not have their own database of radio amateurs in their area, but there's a chain of things that negate that apparent ignorance in reality.
1. A condition of your radio license is keeping your name and address up to date with the FCC.
2. The FCC considers the callsign->name/address database public information.
3. The database is available all over the place. For an example, go to qrz.com and put in any valid callsign. You'll get the callsign holder's last known (supposedly current) name and address along with GPS coordinates, grid location, time zone, etc.
This is one reason many privacy advocates who are also into preparedness will buy amateur radio equipment for use if the SHTF, but not get an FCC license.
To elaborate on the address requirement, the FCC mandates that the address given be the actual physical address of your "primary operating station." A PO Box won't fly.
Also, the amateur service is considered "self-policing" in that the FCC and LEO will enforce the law, but they don't typically actively patrol for violations. All too many amateurs get their kicks from trolling callsign databases and reporting people they know are operating from a primary station that doesn't match the FCC address on file.
Finally, many amateurs are very good at direction finding. There are lots of "find the hidden transmitter" contests held every weekend all over the country. If you are operating an unlicensed station and attract the interest of the locals, chances are close to 100% that you will eventually be found and reported.
Who needs big brother when the sheep are willing to hand you over to the wolves themselves?