A cord that was manufactured with ends cannot be fixed using a listed plug kit because the cord is only listed for use with its original plug. Trimming to install a plug kit is modifying the material away from its listed purpose.
how is that enforceable? For example, I purchase extension cords with molded ends as well as bulk cable from a certain manufacturer. The cable in the extension cords and on the spool is exactly the same, with the same UL number on it. If I take an extension cord, cut the ends off and install hubbells, or if I cut a length of cable off a spool and install hubbells, I end up with exactly the same thing.