As an attorney with contest-law experience, I can say with certainty that the folks here who mentioned making it a game of skill, rather than pure chance, are correct.
While US States do not require this, many require other unusual things, and some even require that a contest, or a contest sponsoring entity, register with the State. Involved in that registration process are assurances that eliminate some of the more gambling-oriented aspects of a contest.
Its a bit like the "no purchase necessary" tag they have on competitions to randomly win stuff in crisp packets in this country (UK). Some legal loop hole to avoid getting mixed up with gambling regulations.