Because some people seem to think that they have an expectation of privacy in public places when nothing could be further from the truth. There is still a huge segment of the population that is largely naive to laws and gives in to popularized paranoia that someone is out to get them. Granted I wouldn't expect the everyday person to have ever read their state statutes much less have any familiarity with things like constitutional or procedural law. I would seriously doubt that it is common knowledge among the general populace that police departments or even something like a repossession agency have used license plate readers as far back as I can remember.
There is a very distinct line though, in terms of privacy. If I am walking down the street, otherwise not committing a crime, I have a reasonable expectation that I'm not going to interact with the police. If I am walking down the street and the police have the ability to do a blanket scan of all persons in the area and determine who has an unpaid parking ticket, I'm likely going to have to interact with the police. And in my mind, there's a line there. I don't think privacy is guaranteed in a public place, but I also think that police should have to do some work to catch criminals. I would certainly support that kind of technology to stop somebody who broke out of jail and is an axe murderer, but the ratio of those to somebody that forgot to pay a parking ticket is so miniscule that it's not worth the tradeoff.
A license plate reader that requires a cop to make a conscious decision to select a license plate is one thing - a dragnet camera that captures every single license plate is something else entirely . I'm also troubled that the article seems to indicate the purpose of this is to stop property crime - does that mean that if I have an out of area plate, or have some very low level drug possession charge or something, and drive through Carlsbad just before a crime is committed, I will automatically be a suspect? Or maybe you now know that about me and pull me over for going a mile an hour over the speed limit just to see what I'm up to. I don't think that's right - and this technology could certainly be used for exactly that type of targeting.
Maybe it's paranoia - but even as a clean-cut white guy in a clean car, I've been pulled over on what could generously be described as a fishing trip just to try to search my car for drugs (which I declined) - I'm not saying that the police are generally bad or corrupt, but I think in their efforts to do good they sometimes are a wee bit aggressive on innocent people, and this provides great opportunities to expand that behavior.