There's a couple strong, imo, arguments against the legality.
For one, average speed cameras are constant, identified surveillance of everyone that drives passed them (as opposed to CCTV, average speed cameras positively identify every vehicle that passes by). Blanket surveillance is bad and possibly illegal, but also it happens to civilians the State has no reasonable suspicion have committed a crime, which is also bad and possibly illegal. Finally, a ticket gets written for someone breaking the law by driving faster than the speed limit... but in fact, nobody, including surveillance, actually witnesses them breaking the law. They just assume they broke the law because timestamps.
I know it sounds silly but it really is a pretty big legal leap to "assume" someone broke a law without actually witnessing the crime itself. You might suggest the timestamps are strong evidence of the crime being committed, and sure, you're right, but then we're back to the blanket surveillance issue. Furthermore, there are actual ways to pass two average speed cameras within a time frame that would "require" speeding, such as by taking a route unaccounted for in the camera's design.
Though in the end the cameras are pretty useless because they put them in the best fast riding zones in the mountains so we just zip back and forth between two cameras practicing the same route a couple times at 3 or 4 times the speed limit and by the time we go past the second camera our average speed is like 20km/h lol. Whereas regular actual speed cameras at the entrance and exit of small towns actually force you to slow down for pedestrian-heavy or dangerous areas.
I've had an argument with some police politician about this, about how putting the cameras in the good riding areas is pushing people away from the safest riding spots in Taiwan with wide shoulders, double lanes, and barriers between oncoming (SO COMMON for cars to violate the double yellow here!!) and into the single lane unlit nightmare roads alongside the big good road, which are impossible to get speed camera infrastructure to (without great cost anyway). Which is driving up deaths so really defeating the purpose.
Norway has the lowest per-capita traffic deaths of any European country. Check out the US's traffic death statistics if you need another reason to be disappointed in the outcomes of our domestic policy.
Be fun to troll the shit out of politicians/elected officials with this. Have someone monitor them leaving one place, then with another car and a fake plate have them get scanned at another location moments later.
The near-real-time cross-referencing is basically a probable-cause factory, and then its on the person getting pulled over to stand up to all the implied threats of violence that the police can bring to bear.
Prior to tools like this, running a dragnet on e.g. invalid licenses was quite a bit more labor intensive, and so there was a built-in grace allowed to people whose only crime was omission of paperwork.