What Blizzard banned was having any type of software or hardware assisting in the multiboxing. Multiboxing in and of itself is not banned, so long as the player follows the rules about no external assistance.
Multiboxing, or playing multiple World of Warcraft accounts at once, is not a violation of our End User License Agreement. Please note, however, that use of all software or hardware mechanisms used to mirror commands to multiple World of Warcraft accounts at the same time, or to automate or streamline multiboxing in any way may result in account penalties.
For more information on our stance about input broadcasting software, see our official forum post.