@Jedic
It's not allowed because you can kill a person by doing it.
My my two shields are one wood, and one metal; the edge of the wooden one has rawhide, and the metal one has a garden hose... and that's the only thing on the edge.
Since the edge is the part you strike with (on a shield), people could literally die.
A shield bash with the face of the shield is not a big deal; it might distract an armoured fighter for a second, but isn't really useful.