It might not be a law .. It might be your state charges extra taxes or they require the sponsor to do something extra in order to be able to include it, sponsor will just exclude your state instead of dealing with the state.
I live in FL (aka the land of cant enter :nono. From what I understand, FL requires any sweepstakes prize to be put in some kind of "account/trust" so that the money for the prize is actually there. This way the company cant say "You Won! but, sorry we don't have the money for the prize"