It is simple. Toolmakers are not responsible for moral arbitration. A convicted felon can legally buy a Craftsman wrench, even if he plans to bludgeon someone with it. Craftsman is not responsible for preventing the convicted felon from buying one of their products. And that's fine.
Selling to a general public is not the same as selling to targeted clients. If you sell computers in a store, and an evil foreign dictator buys it from there, you are not as responsible as if that same foreign dictator approaches you and asks for a delivery.
Even selling to a general public is nuanced, as selling toothbrushes is not the same as selling automatic rifles. even though both can be used to kill people, one is optimized for keeping your teeth clean, while the other is optimized for causing injuries.
The difference in this case is that a retailer has no reason to believe that a random member of the public is going to use merchandise to bludgeon someone. However if a person comes in the store and says they are stocking up for the murder they are planning, you should not aid them. This may even be criminally aiding and abetting!
