Mostly it's people who are upset at being thrashed, and knowing that they have no way to remedy it. I'm sure that they're used to situations where they get ganked, then they wait for an opportune moment to get their revenge, but instead they just wind up walking into 5 lightning bolts over and over.

They assume that repeatedly complaining will work, because they think it has in the past. This is, in nearly every case, incorrect. Correlation is not causation. Just because a lot of people whined before a particular change was made, does not mean that the whining was the reason the change was made. I will say again-- do not expect to see multiboxing forbidden, ever. At the very worst, Blizzard may tinker with class mechanics if they feel that a particular ability is too powerful when used in multiboxing, but that is about all that they'll do... and don't expect them to do that often, either.