I think the difference is that a knife is more or less used for what the manufacturer advertises it for.

Something similar has happened with gun manufacturers regularly. It's relatively easy to make a semi-automatic user-convertible into an automatic weapon. But selling your rifle with instructions like "we absolutely DO NOT RECOMMEND cutting this specific notch off of the trigger group with a hacksaw BECAUSE THAT WOULD BE ILLEGAL" has not been appreciated by the ATF or our court system.