Wikipedia claims[0] that this is not allows in California, though the citation has suffered link-rot.
I think enforcing speeding limits could also discourage the use of things such as ez-pass, which may very well result in more revenue, compared to the same tolls collected in cash by real people. So it could hurt the revenue stream from tolls.
[0] https://en.wikipedia.org/wiki/Speed_limit_enforcement#Averag...
Just a guess. But have you ever seen a local government turn down money in order to preserve civil liberties? Pretty rare these days. So there must be some sort of ulterior motive.
They are useful for catching speeders in a relatively short stretch of highway, but useless for medium-to-long stretches. Even if you constantly drive at 20km/h over the limit, the average can easily dip below the limit if you stop at a service station for a few minutes or if there's heavy traffic at some point. And if the highway in question doesn't have toll booths, the police will have to install a bunch of expensive cameras at every single interchange.