Based on lawyers recommendation. Anyone with half a brain knows there is no reasonable way they can keep underaged people from access their website, but that won't keep someone from suing them if they find jr. browsing it. This is a modicum of appeasement. I have requested one Michigan brewery to change from entering a date to allowing a choice between "21 or over" and "under 21". This made it much easier on my smartphone.
Trust me, the brewery was clueless as the rest of us about how this did anything it was supposed to.
And seriously, I've never seen a brewery website that wasn't suitable for anyone of any age to browse. It's not like they say something like, "Careful kids! This S$%t get you F$%#d up!"
There isn't any law requiring it, it's just to minimize potential lawsuits.