I imagine no-one wants to be on the receiving end of "You are accused of actively encouraging Iranian / Russian / <insert other sanctioned state here> hackers to identify exploitable security vulnerabilities in appliances owned and operated by Americans; how do you plead?"
Wouldn't the legal definition of "restriction" also include the laws covering computers etc?
A technicality but one could argue that if the law is the only barrier to exploiting something then the vulnerability needs to be fixed and proven, which a US citizen can not do.
Can we make this a condition of giving any prizes, rather than of entry to the competition? This restriction affects literally 200 + million people.