You can also heal yourself by not selecting a target if that's really issue. There's probably just a line of code that says that if you don't have a legal target for the heal (i.e. a PC or NPC) it defaults to yourself. It wouldn't take much to change an enemy to a legal target when they are of this type (or all enemies for that matter, but heals won't affect them since it by default does no damage). There is already a modifier for healing (seen in skill such as mantra, convalescence, defiance, etc.) so the modifier can by 0 for normal enemies and -1 (or what ever number) for undead.