Alcohol is allowed in sanctioned leagues if allowed on the property. I personally see no reason why the rules should be different for leagues than anything else if the rounds are going to be "real."
Yeah, I am of this mindset. The rules of disc golf are not prohibitive of fun, in any way. If one is going to skirt rules, I think sanctioning is not going to curb that. Most leagues I play, everyone is playing by PDGA rules. The drunk, stoner guy is not a ton of fun, regardless of sanctioning.