Maybe spells are affected, but not abilities. The fire bolt spell from the item has a cooldown and no mana cost, so I think it is an ability rather than a spell. The game still treats them slightly differently.
EDIT: Though that does not explain why the Blind from the Darkling Cloak doesn't work. I don't think that has a mana cost either...
I think some of the spell or ability giving items are just bugged in some way so they don't give the spell or ability.