Inquisitors can't use their special abilities on demons because... they are demons, it goes against their faith.
As for Necro Call, you have to look at the available undead unit types to see if said spell would work on a particular dead stack. That's why you could necro spiders since there are undead spider units, while you can't necro snakes.
As for necro on demons, I'm assuming its more of a "fantasy logic" rationale; they are technically one step lower than undead, being already in hell and all that.
I'm just glad the developers allowed the use of the Resurrect spell on them since Demoness (with Anga's Ruby and Xeona) is my favorite unit.