The designs are copyright protected. Here is Utah and other states may be the same.
that fraudulently altering or counterfeiting a registration card, license plate or permit issued by the Motor Vehicles Division is a second-degree felony under Utah law, carrying a penalty of 1-15 years in prison and a $10,000 fine.
And while many of the violators eventually have the charges reduced to a misdemeanor level, it doesn't start out that way. The felony classification results in the immediate arrest of violators, which has caused some upsetting and embarrassing experiences for average people who would otherwise never find themselves in jail.