I was also stopped because our state only has one tag. The cop told me that every state has two license plates.
I looked at him with a puzzled look and laughed and said okay if you say so. He said I'll prove it. Okay show me I said. He was stupid enough to get on his radio and ask dispatch. I heard the dispatcher ask him if he new what he was asking? He said he wanted to know. Anyway the cop gave me my license and never said another word, got in his vic and drove off.

I think Iowa State cops are a little more informed on things, but small town cops in Iowa apparently just don't see many single-plated vehicles. Iowa has no bordering states which issue a single plate (although legislation was introduced in Jan 2011 to have Illinois move to a single plate system).
There are
19 states which issue only a single plate, with Arizona, Arkansas, New Mexico, Kansas, Louisiana and Oklahoma being the only ones that are west of the Mississippi.
The others with a single plate are: Alabama, Delaware, Florida, Georgia, Indiana, Kentucky, Michigan, Mississippi, North Carolina, Pennsylvania, South Carolina, Tennessee, West Virginia.
Proponents of dual plates say it makes law enforcement easier ("We don't think this is an area in terms of law enforcement that should be skimped," said Illinois Secretary of State spokesman Dave Druker), but the states (like Indiana) which have moved from two plates to one say that's not really true and that single plates have no negative effects on law enforcement.
Having two plates is silly and a little stupid, if ya ask me, which nobody did. All a second plate does is add cost to production, use additional resources (metal, plastic, paint, sealers), create
double the amount of waste and do not give police any extra tool in law enforcement.