The tech exists to do it on-camera (Ubiquiti among other vendors claims on-camera LPR), but I figure they're doing whatever is most battery-efficient since the ones I've seen all have solar panels.

I do wonder what they do on busy interstates though. Some of the ones I've had to go past would basically be sending a live video feed - LPR matches as text would save a ton of bandwidth and maybe even power compared to a cellular radio always transmitting.