A shield can inflict damage just like a sword can. Encryption hiding the details of a murder plot can be just as crucial as an exploit that reveals a murder target's location.
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.
I looked into this because I was intrigued, turns out convicted felons in the good ol' United States are not allowed to purchase/wear/own body armor [1] (barring explicit exceptions).
I wonder what the rationale behind this federal law is. Does wearing/purchasing armor indicate that you, convicted felon, are simply up to no good once again? Is it one of those "you don't need ~~privacy~~ armor if you don't have anything to hide" things?
Because the state wants to make their ability to use force against rule-breakers as strong as possible. Some states even make the use of body armor while committing one crime it's own separate crime.