For those that don't know, Blizzard is using 'undead can't be mind controlled' as a trope from Dungeons&Dragons - where undead creatures are immune to all mind-affecting magic and effects (they're usually classed under the enchantment/compulsion magic school), and are usually under the control of their creator up to a [level] times [X] limit. Going over the limit usually creates 'free-willed' undead (or they can be released by the creator). Warcraft's lovecraftian themes imply the compulsion kind of control.
D&D undead can be temporarily controlled by using some (necromancy class) spells by manipulating the negative energy that powers them.