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...
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.
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.