Modern unmanned aerial vehicles like the Northrop Grumman RQ-4 and the General Atomics MQ-1 cost in the millions. Cessna and helicopters do not.

Granted there are cheaper UAV out there, and generally its unlikely they could carry a payload sufficient for the level of surveillance necessary for traffic enforcement.

BTW - my intention isn't to advocate for drones, just to question the rationale of making a distinction between manned and unmanned surveillance, when in fact it might make more sense worrying about being aerial surveillance more generally.