The first is that significant speed differentials are extremely dangerous. Uncalibrated speedometers are very common and very inaccurate. Anything that causes guaranteed enforcement is going to reduce safety because people will start religiously following their speedometer instead of the normal flow of traffic, so now you have people driving all different speeds than one another based on how badly and in which direction their speedometer is miscalibrated. On top of that you have a situation where anyone who has made a stop for coffee or gas is exempt from the timer because of the stop, so you put a number of people driving 15MPH over the limit (because they stopped and aren't subject to the timer) on the same road as people driving 15MPH under the limit (because they haven't stopped and their speedometer is inaccurate) and you're going to see more collisions.
Then there is the pragmatic problem. Speeding tickets are big money because the law is enforced sporadically under unpredictable conditions. To generate revenue, enforcement has to be rare enough that it doesn't induce compliance. And the proposed scheme is entirely deterministic. Everyone who doesn't make a stop will discontinue their speeding between toll booths -- which is ostensibly the intended result, except that it means the scheme will generate no money and will deprive the government of existing ticket revenue that would have been generated on that stretch of road. Meanwhile it reduces the need for police presence, which means you'll have to fight the law enforcement unions to boot. It's not at all surprising that there has been little interest in implementing it.