Ok, I see what you mean. First, it is up to member schools to institute drug-testing programs, but the penalties for a positive test are not administered at the school's discretion (
http://www.ncaa.org/sport-science-institute/topics/frequently-asked-questions-about-drug-testing). Secondly, I take it what you mean is that it is up to the school's discretion whether or not to report the violation (e.g. Bo's policy). So it is wrong to say that every school is held to the same standard, because a school has the discretion to (a) test its athletes and (b) report those tests publicly (though this is just an invitation to be investigated and sanctioned, cf:
https://www.cbssports.com/college-f...homa-state-for-violating-drug-testing-policy/).
None of that means the NCAA is not a legal body. Of course it is a legal body--it has rules and the authority to enforce them, and every school that is part of the NCAA is subject to those rules. What you seem to be taking issue with is the lack of executive power and/or oversight the NCAA has. Yes, violations must be self-reported or come to light by other means (reporters, a criminal investigation). That does not mean the NCAA does not have rules and does not enforce compliance. (How else do you make sense of the fact that the NCAA imposes sanctions?).
Anyway, if the worry is that schools located in states where weed is legal will gain a competitive advantage, the solution is rather simple if you're a coach residing in a state where it's not: don't administer drug tests.
Perhaps there is a worry that drug-tests will be mandated by the school, or by the state provided that the school is a state entity? I guess that is a genuine worry, but as you've noted it is hasn't really been an issue up to this point because coaches seem to have no trouble instituting their own punishments without the findings becoming public knowledge (as Bo did with Gregory).
Lastly, I don't think most college coaches
want their players smoking weed, and so it will only be a competitive advantage for those coaches who don't care or who look the other way. Is that any different than things are now?