I'd argue this is more of a concession to bots than anything else.

When they originally removed /follow from battlegrounds they said it was to get rid of bots. Really it only gets rid of shitty, most-obvious bots, and multiboxers. I'm sure they could pretty much care less if multiboxers are in battlegrounds, if it weren't for random complaints and people referring to multiboxed toons as "bots". I don't think they're really that concerned about less obvious bots -- as I said back in 2007 when I was working with Blizzard's Warden (anti-cheat) on a regular basis and knee-deep in the legal BS between Blizzard and Glider, "quite frankly, if a cheater does not appear to be cheating to the other players, then clearly no harm is done to the game". I can't speak for them obviously but I personally met and talked to various members of the Warden/Risk team at Blizzard and they didn't exactly disagree with my statements from the time. (They did, however, say that Prepared running so many accounts would become a problem for multiboxers.)

So instead of spending their time and energy dealing with the actual bot problem, they're taking the route of reducing the number of characters who appear to be cheating to the other players. This is a much easier and less costly solution for them, especially considering that with respect to Warden and detecting bots running on players' PCs, their hands are somewhat tied! (They can't implement scans that are particularly invasive, and bots are pretty good at hiding from them.) One of the only remaining defenses they have is legal, and with bot developers not in the US (say, "for the sake of example", in Germany) the legal team also has their hands somewhat tied.

The practical effect of this is that the more human-looking bots become more effective, and the profits of said bot developers is increased (people have to use those bots instead of the shitty ones) -- and there is the concession to bot developers. Blizzard is, blatantly I would say, choosing these bots over multiboxers.

And I am certain that many people who would otherwise be multiboxing and following the rules, will be botting instead and violating them. In this scenario, that is a win for Blizzard.

They have to rely too much on player reports, and they're effectively reducing those. Meanwhile, they can say they're doing something about the bot problem, they can still show that they are banning people (a la reports), and they can still rake in cash from selling accounts and subscriptions to a pretty sizable population of botters who are often happy to repurchase the game.

Long story short, I don't think that cancelling your multiboxed accounts will affect change on this issue. They will make that money back from botters -- a much larger population than multiboxers. However, we could maybe put together a formal legal complaint in a civil court to argue the issue, and maybe in the process force them to confirm in public records that they are not doing what they claim to be doing against bots.