Earlier quoted context omitted.
In most cases an ambulance is required much more urgently than a tow truck. The cops can call the towing company they prefer, once they're at the scene. There's no need to complicate the 911 system with towing, unless it's important that kickbacks from towing companies be paid at a high level rather than directly to individual cops.
Cops at the scene should not be looking up phone numbers for this week's roster of towing companies. That is handled by the central police dispatcher. Who also dispatches SWAT, bomb disposal, fire, hazmat, linemen for downed electric wires, animal control for welfare checks of sick people who have pets, etc. 911 is just one part of an integrated emergency response system.
Even out here in the rural hinterlands, 911 operators and police dispatchers are different groups. The cops can call the tow truck however the hell they want. The 911 operators still shouldn't care about the dogcatcher.