I recently bumped into the same problem using UDB 403.
I've searched all the tables in the database, but there is no table where I can possibly fix this.
So I think it might be a mangos problem.
The problem probably is due to the 'Unarmed' skill. I tested wearing a fist weapon on a DK and a paladin and both can wear it when both should not be able to wear it.
It could be, as stated above, that unarmed means you can wear fist weapon, but I don't beleive that because:
a) why is 'fist weapon' written in red
b) why, according to below (just a few) sources, DK cannot wear fist weapon on bl!zz servers
some forum post
old wowhead
wowwiki
edit: did some more testing, when hitting with a fist weapon equiped and not max skilled, you improve your unarmed skill. so it realy is related to unarmed on mangos+udb/ytdb but it should not according to the above mentioned sources