So I go to Wal-Mart today and notice a guy getting carded for beer, but not for the 6-inch knife he was purchasing.
I voiced my amazement at how weird that seems. Another guy behind me cuts in, "That's nothing... when I came here last time I was purchasing lighter fluid, a knife larger than that one, and a copy of Robocop. They bugged me for ID over the movie".
There's something wrong with a system in which people can get canned for failing to ID customers over alcohol and R-rated movies, but not stabbing weapons, maiming devices, and the method to start large fires.
You must be 18/21 to harm yourself, but the means to harm many others comes with no restrictions.