The reasoning behind this is, of course, not completely understood. It is possible that this is the latest strategy in Blizzard's unending battle against bots, which might occasionally, though definitely not always, use following as a way to help avoid being afk booted.
However, it is more likely a decisive step to making it far more difficult for multiboxers to dominate in battlegrounds. While not against the terms of service, multiboxing in random battlegrounds is a big problem for many players, when five DKs or five elemental shaman are running around causing havoc, it's hard to enjoy yourself. We've all been there, and it is definitely annoying, although they can sometimes be taken out with a group effort.
Based on the latter reasoning, it's probably a good thing for the overall enjoyment of casual PvPers, who vastly outnumber the people who want to five-box their way through battlegrounds. Anything that makes random battlegrounds more enjoyable is just fine by me, but should Blizzard have given multiboxers more warning? Or is this more part of the anti-bot measures, in your opinion?
Updated: Hit the break for Blizzard Senior Community Manager Bashiok's input.
Bashiok's comments clarify Blizzard's actions, and it looks like battleground multiboxers are an unfortunate casualty of the ongoing war against bots. Again, it's likely a numbers game, and the quantity of players irritated by bots is probably far greater than those who choose to multibox in random battlegrounds.
However, the PvP multiboxer is likely to still have the arena at their disposal, and following Bashiok's remarks about pre-determined group play leaves WoW Insider wondering whether a revocation of this change would also be appropriate in rated battlegrounds.