It's parents, culture. The science really backs not touching alcohol till 27 or so when brain is fully developed... I know, I know... Don't shoot the messenger:
"Wisconsin permits the consumption of alcohol by minors, provided they are being supervised by parents/guardians/spouses."
EDIT: Notice how vaguely that is worded compared to other states that allow underage exceptions... Other states individualize it. I guess in Wisconsin, underage drinking is allowed at parties, just gotta be supervised, right?