This question was answered for me by a Maryland DOT person a few back during a tour of one of their operations center. The upshot is that the intended purpose of EZ-pass and other toll systems are for improved traffic flow, there is a lot of DOT involvement and the DOT wants to encourage folks to use them.
If those toll systems were to be used for speed enforcement, people would be discouraged from using them, which hurts the DOT's objective. Furthermore (and this is probably the bigger reason), since the DOT is deeply involved in deploying and monitoring the system, they would be subject to subpoenas by offenders challenging their speeding tickets and the DOT absolutely wants no part of that.