Because of idiots like this:
http://www.fairfaxunderground.com/forum/read/2/85780.html
It's an age restricted item. The merchants reserve the right to refuse a sale to anyone for any reason. If they get caught selling to an underaged person the ABC board may revoke their ABC permit for a while and levy a fine, both of which are very costly to them. This is something that they would rather not deal with.
There are three basic ways to deal with this:
* Either make sure everyone on your group has their ID when purchasing an age restricted item
* Shop somewhere else
* Drink water