I'll try to explain as to why it's against the Terms of Use. You're not using game mechanics when you're doing this, you're abusing that there's latency between you and the server, to achieve something that isn't supposed to work.
Although, if you succeed in doing this, the chances of anyone reporting you for it, is extremely slim if non existent. But given the hatred that Alliance and Horde has against each other, in the event someone does report it. It's completely up to the Game Master, which
might not even care due it being completely harmless.
IntendedThe enemy is only friendly during the duration of the Mind Control.
The moment Mind Control is interrupted or canceled, the enemy is returned to his original faction. Rendering all buff/healing attempts void.
RealityThe packets your client sends to the server, which contains the information that your character is attempting to buff a enemy character is received before the data containing information regarding the interruption of the Mind Control. Which causes the server to respond to the first packet, which in turn buffs the character instead of giving you an red screen error. Shortly after this the server receives information that the Mind Control is interrupted and changes his faction status accordingly.
PreventionIf the server received the interruption of Mind Control packet first, you wouldn't be able to buff the enemy since his faction status would had been changed already.
Had they implemented (if) statements to check whether the caster of the buff is the same as the one who's currently channeling the active Mind Control then this wouldn't be possible. -
If they did this, further latency would occur when casting spells.Therefore using this would fall under the Terms of Use of Nostalrius - Abuse of in-game bugs;A software bug is an error, flaw, failure, or fault in a computer program or system that causes it to produce an incorrect or unexpected result, or to behave in unintended ways.