Even toll roads in the USA are enforced by photographs of licence plates

No one enforces toll collection with photographs of license plates as
the primary system of enforcement. It supplements transponders.

We may have a problem with the word "enforce". While it's true that
transponders are the primary means of collecting [US highway] tolls,
supplemented in many cases by a cash-lane, what the cameras are doing is
enforcing a regime where people need a transponder (or to use the
cash-lane).

In some areas yes, in others, no. In Florida and Texas, and on the
407 and A25 in Canada, there is no cash lane. If you don't have a
transponder, they photo your plate and send you a bill. That's not
a violation unless, of course, you don't pay the bill.

In California, SR 91 has tolled express lanes which you can only use
if you have a Fastrak transponder, and driving on them without a
transponder is a violation. The regular lanes of the same road are
free.

R's,
John