Any private person can collect license plate data with a computer too. Surely that's not illegal? Imagine thousands of hobbyists doing this and building a database of when and where cars are moving around the city. If this data collection is only allowed on "small scales" but when people combine or publish their records it becomes illegal for the group (but not the individual members?), that would become a pretty hairy legal gray area.

Also cars are a massive cause of death and heavily used in violent crimes so it can actually be useful to track them, unlike trying to catch near-non-existant terrorists.