Gold farmers also increase the amount of subscriptions that the game has, more money going into the developers... I don't get why they fight it so much.
To me - its the worst business logic I've ever come across, and games that have these microtransactions already involved will be the ones who come out on top.
Smart businesses don't think in raw, bulk numbers like that. Not all subscriptions are created equally.
Consider why people get banned from the game at all; it usually involves something that negatively impacts the play experience of other players. Sure, Blizzard loses $15/mo if they ban Player A, but they stand to lose many times that amount if they let Player A run around being a jerk to Players B through Z. One account termination due to the actions of Player A costs the company the value of his subscription.
The same essentially applies to gold farmers, in a more indirect way. Early in WoW's history gold farmers impacted other players simply through economic inflation and crowding out resources. Some resources were almost impossible to farm for yourself because of the ubiquitous presence of farmers. Blizzard banned these farmers because of these negative impacts alone, and the money it cost them to do so was likely saved in preserved subscriptions and reduced support costs.
This argument is far more pointed now. Gold farmers still inflate the economy, only they do so through ill-gotten gains obtained by hacking accounts. The support costs are many times greater than before, and are far more likely to result in account termination. Losing one's account, even if you can get it restored, is the single most frustrating event that can happen to a player in a game where progression is a foundational goal.
So Blizzard is saving a lot of money by working to prevent gold farming.
Also - the big question - why would this become illegal? People do what they want with their money. If Blizzard was smart - they'd offer Gold at a price matching the market and get a cut on this. They've already ruined WoW four times over. Anyone who's played since the beginning can tell you how much more enjoyable it used to be.
It's disingenuous to claim that "anyone" from a category designed to exclude people who might disagree with you will agree with you. It's also demonstrably false, as I fit into your category and find that the expansions and patches have markedly improved the game.
Your second disingenuous argument suggests that having already ruined something, there's no reason to avoid ruining it further. This is frankly silly, as the mistakes of Star Wars Galaxies can attest.
More importantly, it's impossible to price match a black market. I've tried several MMOs which allowed RMT transactions, and was not surprised to find that even there the black market currency farming/selling still thrives. Even if Blizzard/other companies offer their currencies cheap, someone will always be ready to offer it cheaper.