There's more to the "missing plate" event than a lot of people know about. There are some virulently nutty types who have espoused a "the government can't legally force you to have a license plate on your car!" idea. There was a nasty periodical called "Anti-Shyster" that had some stuff about the idea. The "missing license plate" can be spun as a continuing "anti-govt." thing - removed on purpose by the "mad bomber" to show his contempt for govt.
"the government can't legally force you to have a license plate on your car!"
I like cops just fine but I don't want to get pulled over every time I leave the driveway.
Fact of the matter is, missing plates are also a trick by undercover agents to have someone picked up without blowing the agents cover.
A number of informants were close to McVeigh. Presumably, agents knew of him as well.