I'd be more annoyed if the mini was actually something new with new stats. Limited edition convention re-issues of common things is normal. Loads of minis companies do this.
The way Lego does their con exclusive minifigs is just a big present to scalpers. But this, or the limited edition Heroscape repaints, or the Games Day minis, or the tons of free Reaper Bones I got at Pax, is just normal con stuff to me. Though I don't know maybe GW does awful things with Games Day minis nowadays.