It's not just parked cars - license plate tracking can (and does) include moving vehicles as well. You don't have to 'park your car in public' to be scanned. Mike's anecdotal incident(s) above show that.
The problem with license plate tracking and retention is not the technology itself. It is not inherently evil. Used properly, it can be a very effective tool for identifying/recovering stolen vehicles.
The problem lies in defining what is proper. Law enforcement has this technology with no checks and balances on how they can use this scanned plate data. Do you trust law enforcement to 'police' themselves?
Without any guidelines/rules, the data (in aggregate form) can absolutely be used to determine where you've been and what you do.... definitive patterns can certainly be identified with enough data points.
And, because the tracking is location-based, even those who you associate with (think civil protests, political activism) can be determined via geographic grouping of scans.
I am not against license plate scanning. I am against unregulated license plate scanning.
And for those that subscribe to the "I've got nothing to hide, so I don't mind" line of thinking, please note that privacy does not mean secrecy:
Why Privacy Matters - Even If You Have 'Nothing To Hide'
Debunking The 'If You've Got Nothing To Hide' Argument