This is an excellent point, and why I point out that they missed a butcher's knife in a purse (not a giant rollaboard bag) at EWR, where they're obsessed with shoes.

The other thing that concerns me with this shoe stuff is how inconsistent it is across airports and even across terminals at the same airport. If they can't achieve even a tiny amount of consistency on this, I'm worried about what other policies (like looking for guns/bombs) are inconsistent and thus insecure. The shoe carnival is in-your-face: we can't miss it. But other policies being poorly implemented are more behind the scenes and so we don't really know what's going on with them.