It could be that they are still targeting an enemy, and then casting on themselves. I assume you're using the Focus, clicking on who needs healing, and then pressing the heal button? Depending on how long you take to press the Heal button after you select a new target, it may not have updated for your slaves yet. So your slaves think your focus is still attacking something, and since you can't heal enemies, they heal themselves.
Or at least that's my theory on your problem. To fix it, just wait an extra second or so after you select who you want to heal before pressing the Heal button/macro.