For underage drinkers though, it IS an ILLEGAL substance. Free invitation for you to come out to UMC/Southern AZ and watch how many people get "tagged" for underage drinking. No joke...if UofA PD catches any UofA students with ANY amount of ETOH in their system (AZ state law says the hospital is allowed to reveal contents of a tox screen, including BAL to PD without breaking HIPAA, but only that info), they can (and occasionally do) get kicked out of the University (if they are in the dorms, they at least loose their student housing) and a few times I've seen them revoke full scholarships for it. The UofA has a ZERO tolerance for ETOH in underage drinkers. PD doesn't have the authority to actually revoke the scholarships themselves, but if they (the ETOH + student) falls under a certain category of offence in relation to ETOH, once the arrest is forwarded to the UofA then it's revoked by the financial aid people at the UofA (built into the "contract" for financial aid).
Also, just FYI, because the legal drinking age of Mexico is 18 (I think, I know it's lower than AZ), AZ DPS (AKA Highway Patrol) actually does set up check points for people returning from Mexico to bust underage drinkers (regardless if they are driving or not). God help them if they are driving...that's a LOT of charges (according to DPS, can be minor in possession of controlled substance, reckless endangerment, DUI [even if their BAL is 0.01 or less, because it's illegal for them to be consuming ANY ETOH], plus any speeding, or MVC related charges, if applicable)!
For people 21 or older, if you have some ETOH but are still "competent" to make a decision (A&O X 4, without OBVIOUS signs of intoxication), then by all means, refuse. But for minors (under 21) who have consumed ETOH, they have lost the right to refuse, because for them, ETOH is an illegal substance, whose abilities to alter mentation have been well documented throughout time (at least several thousand years).