And you will be jailed should you murder someone with a shield. But since the primary purpose of a shield is defense, it's assumed you will not use it as a blunt weapon. Buying a two handed longsword will definitely get you on some list, however.

Stop trying to find gotchas. Weapons (read: items whose primary goals are to inflict damage, maim, kill, injure, destroy) are and should be regulated.

The point of OP's original comment is that it isn't enough for the seller to assume that the customers will use the product in a legal manner. Once you start holding the seller legally liable for use of their product, the primary purpose of that product becomes irrelevant. All possible uses must now be considered when selling a product.

> A shield can inflict damage just like a sword can

That's your mistake right there