Well, it could be the intent that Police weren't happy with. After all, if your not going to speed, you don't need a jammer, so the logic says that if you have a jammer fitted to your car, there is an intent to speed, regardless whether you are speeding or not.
The guy could have easily been doing 10 mph below the speed limit, but because the camera operator could not get a valid reading, then there is reason to believe that measures have been taken to obstruct a measuring device. Add to that the rude gestures, this gives Police the necessary impetus to investigate.
Whether we like speed cameras or not, they are here to stay and have government backing for their use, and any attempt to disrupt their use is criminal in nature. I personally don't agree with their use, prefering our highways to be Policed properly by humans, but that doesn't mean I would douse one with petrol and set light to it, which is a regular passtime in the area I live.
The way I see it is simple. If you commit a crime, you should be discrete, not shout it from the rooftops. This guy shouted it from the rooftops, so gets everything he deserves.