Um, it's more like 20 states which don't require both plates:

From Wikipedia:
"In the U.S. states of Alabama, Arizona, Arkansas, Delaware, Florida, Georgia, Indiana, Kansas, Kentucky, Louisiana, Massachusetts (most plates issued prior to 1988 - see note below), Michigan, Mississippi, New Mexico, North Carolina, Oklahoma, Pennsylvania, South Carolina, Tennessee, and West Virginia, license plates are only required on the rear of the vehicle. Some vanity and specialty plates in Arizona and Kansas are issued in pairs, but only the rear plate is required to be displayed; the optional front plate is something of a bonus for paying an extra fee for the plate.
Massachusetts is a unique case for license plate mounting. Before the introduction of the current "Spirit of America" base starting in 1986 for commercial vehicles and vanity plates and 1988 for all other vehicles, the state issued only a single green-on-white plate to be mounted on the rear bumper. With the current-issue base, two plates are issued, to be mounted on both the front and rear bumpers. Since all license plates issued since 1978 are currently valid, most passenger vehicles registered in Massachusetts display two plates, and those with the older green-on-white plates only display one."