I don't think the "sober mind" clause is telling people not to drink what they want to drink or smoke what they want to smoke or anything like that. If it said that, I think we'd all want it gone.
As I understand it, it's just saying that one can't make the oath while drunk or high. The oath is a serious commitment and requires that the oath taker fully understand what it is they're committing to. That's my interpretation, at least.