Playing Devil's advocate here....I don't flash my lights for LEO's using radar/laser/Vascar/stop watch/abacus for two reasons. The person they stop may be convinced to slow down a bit, preventing something catestrophic from happening down the road. Another reason is that the person they stop may be wanted for something and this is their ticket to jail. I do flash my lights at oncoming traffic if there is a hidden issue around the corner like an accident or debris in the road.
Any law can be questioned about constitutionality by someone through the court system assuming time and money are available. Headlight flashing for radar traps has been brought to the courts all over the country in different states with differing results. I don't think that any of them made it to the Supreme court, though.
I still think it would be an interesting case to watch, though.