From what I've seen and heard over the years, most states (probably all) first require that passengers can only be carried on motorcycles that are designed and equipped to carry a passenger - seat, foot-pegs or floorboards, handles or strap to hold on to, etc. Secondly, the passenger must be large enough to be able to place their feet on the pegs.
Also, while not necessarily passenger-specific, the laws uisually prohibit carrying anything in a manner that may interfere with the rider's ability to safely operate the vehicle. This includes carrying a passenger on the gas tank or seat in front of the operator.
Carrying a passenger that can't reach the pegs means that the passenger will be unable to maintain a firm seat should the bike make any sudden or quick moves (swerves, hard braking, etc). Even with a good grip on the rider, there is a good chance the passenger could slide off. It ain't worth the chance.
Don