In an upcoming content patch (2.1.0) Warlocks will be able to use their succubus to seduce many humanoid creatures which are currently immune to charm effects.
To clarify, the creatures do too much damage to allow them to be charmed (whether it be by a priest or by an engineer). Seducing them does not threaten the balance of the dungeon.
This will mostly effect heroics. And it only makes things able to be seduced -- it doesn't make anything immune to mind control that was previously charmable. It's not a nerf for priests. It's a bonus for Warlocks.
That makes some sense of why these mobs were no-seduce, too. And, as a healer, if there's one thing I love more than mana, it's crowd control.