The rules for shields say that you "can bash an opponent with a heavy shield, using it as an off-hand weapon". Some feel that since they mentioned that you can bash as an off-hand attack (the usual way to use a shield) they must have meant that you can't bash as a primary attack.
That's not supported by the RAW. A shield (when used to bash) is a weapon. The rules for weapons say you can use a light or one-handed weapon in either the primary hand or the off hand. Nowhere does the shield description say that the regular weapon rules don't apply to shields. (Nor would it make any sense if they didn't.)