Former law enforcement officer here. I can say that when we conducted surveillance, we weren't allowed to use equipment that wasn't common for normal people to use. So for example, I couldn't use some sort of long-distance microphone to overhear a conversation between two people in public because a normal person wouldn't have that same type of technology. I think this applies in this context to say that a cop just scanning and logging every single plate that traverses a certain area is outside the bounds of what we'd consider lawful for normal surveillance. Alternatively, if a normal person would just sit down and write down every single car description and plate that passed by their house or any other public area, a cop doing the same thing would be reasonable. The scale of modern data collection for police is whats dangerous to individual privacy rights.

I think the issue of what your average citizen can or cannot do is a relatively minor.

The bigger problem is law enforcement's use of private business as end-around to dodge 4th amendment.

Google and Apple, Verizon and AT&T, auto manufacturers, Flock, and every other business that scoops up our data has a trove of information that would be unquestionably illegal for law enforcement to collect directly due to 4th amendment protections against government mass surveillance. So, instead, the businesses conduct the mass surveillance and then law enforcement gets to access that information -- either with a warrant as is the case for Google and Apple, or with some sort agreement as with Flock.

Businesses (or individuals) that deliver surveillance data to the government, compelled by warrant or otherwise, should be subject to the same constitutional restrictions as the government itself.

That's not a good argument against ALPRs because citizens can theoretically set up an ALPRs too.