Sounds to me like he was riding that close to see for certain if it was a sticker on your windshield, since that's what you got a warning for.
And every US district court of appeals has ruled that it is legal to run plates without a reason to do so, since A) plates are put on your car for the sole purpose of identification, and B) you can't have an expectation of privacy for something that the state owns the records to. IE: the registration information is owned by the state, so you can't expect that law enforcement wouldn't be allowed to check records owned by the government.
But in your case it wouldn't matter since he had a reason to stop you.
Pittsburgh Pirates- 1st place NL Central 7/15/11
They've got us surrounded? Good. Now we can fire in any direction, those bastards wont get away this time
An amateur will train until they get it right. A professional will train until they can't get it wrong.