Ok, so there's no Big XII rule prohibiting alcohol. It's up to each member school to enact alcohol restrictions. Per an MU article...
"The Big 12 conference currently does not have any members allowing the sale of beer or alcohol within campus-owned stadiums. MU allows for the consumption of alcohol inside the private suites, the press box and the Tiger Lounge. For those of legal drinking age, tailgating is permitted in designated areas surrounding the stadium and parking lots, according to mizzoutailgating.com."
“Here at Mizzou we only serve alcohol in the football press box, which is a controlled area,” she said. “Although there is no rule preventing schools from selling alcohol at sports events, it is highly recommended from the NCAA as at all NCAA Championships there are no beer sales.”
Per a WVU article...
"The University of West Virginia is set to join the Big 12 this summer, and it will bring a decision unheard of in this part of the country with it — allowing alcohol sales at its home football games.
If this succeeds, other Big 12 schools like OSU may feel pressure to follow suit.
On June 3, the West Virginia Board of Governors voted 10-5 in favor of allowing alcohol sales at home football games. Board Member Tom Flaherty said a part of the decision was based on the prevalence of alcohol already seen at games.
“Anybody who thinks there isn’t alcohol in Mountaineer Field is either naive or blind,” Flaherty said. “It’s been going on at every game. It is there. This is a step forward to try to control that.”
West Virginia will take precautionary measures with the sales, such as not allowing points of sale anywhere near the student section, increasing security and cutting off alcohol sales at halftime each game. WVU Athletic Director Oliver Luck said this decision was part of a plan to facilitate a friendly environment at the games."