Why cant they use healing potions?
Stupid rule stupid rule stupid rule.

They can if you install TOB

I don't know anything about ToB, but if you look at the kit description it says "Can't use any magical items except weapons and armor"...
I think with the wizard slayer, they were going for a warrior who hates magic and would therefore shun magical objects, lest they be a hypocrite

I think it's very much acceptable that they finally allowed wizard slayers to use healing potions with ToB installed. after all, healing is priestly magic and not linked to wizardly magic. this is the same reason that clerics can use their spells in athkatla without the intervention of the Cowled Wizards, whereas wizards cannot. in my opinion, wizard slayers should also be able to use Protection scrolls as well (this too is priestly magic.)