There is. A law which requires carriers to provide sufficiently accurate cell phone location data so they can send it along with 911 calls.

https://www.fcc.gov/consumers/guides/911-wireless-services

"Phase II E911 rules require wireless service providers to provide more precise location information to PSAPs; specifically, the latitude and longitude of the caller. This information must be accurate to within 50 to 300 meters depending upon the type of location technology used."

Not the law I'm guessing you were after, huh?