[Joseph Jostar]

I think it's something along the lines of bare fists being a 'weapon' you're counted as having equipped when you have no weapons equipped (with, of course, the fist type)... So when you put on a sub-weapon, you no longer have your bare fists, and therefore no fist weapon. That said, why would you ever want to use your bare fists anyway, since fist weapons get felidae/shaitan claw damage added to them anyway? (If it's trouble finding them, arena weapons lady sells the starter glove for 25 Murai)