More enemies "magical" abilities are focus like than independently providing ability.
If this was the case, wouldn't the ability be written up as part of the monster?
Why did you assume that lightning bolt he was firing was from the staff.
You must have read my mind, as it's a particular shock-staff* in KotS - wielded by a Hobgoblin, I think - that's always been my go-to example for this.
I assume the shocking is being done by the staff as that's how the write-up parsed when I read it.
And you could use the same goblin bleeding blast wand if you were sanctified by the goblin priest every full moon while they sacrificed some innocent... hey want to become the monster?
Sure, and if that was given as part of the story in the module as to what makes that item tick that'd be great. But is it?
If it's not given in the module, how many DMs are likely to add something like this to an item's backstory? I'd wager very few indeed.
I mean players have lots of abilities that do not come from devices... why assume NPCs are vastly different
When writing up an opponent for a module the writer really needs to take a few basic things into consideration. The shock-staff, for instance: can the Hobgoblin still do (or even attempt?) the shock effect without the staff if he gets disarmed? If yes, then this needs to be made clear. If no, then the ability comes from the staff itself unless some other explanation is provided - which it isn't.
Making an enemy magic item temporary only is also an option. So that its useful for helping get passed a certain barrier ... but its energies are supported by some other feature the NPC had.
Yep, this works sometimes...but to do it every time would be a bit much.
* - I ran KotS (converted to our modified 1e system) in early 2009 and the party came out with that shock-staff in their treasury; and ten real-world years later that thing is still booting around in my game world somewhere (though I can't for the life of me remember which character has it these days).