If you've ever played the game, you get harassed by gold farmers all the time. It has been a constant arms race between WOWGlider and Blizzard, and in the times that WOWGlider is ahead, gold farmers are a significant annoyance in the game.
And therein lies the problem: this isn't a conceptual exercise, it is a problem in practice. Blizzard will never be able to ban 100% of WOWGlider users, and they will never be able to make their software 100% secure to this stuff. So, they have to attack it from all sides: they ban cheaters (I personally know someone banned after running the WOWGlider trial just once), they secure their software (I'm sure you've heard of the Blizzard Warden), and they go after the people who make this software.
Like I said, I agree with everything you said in concept. But in the real world, it isn't so easy to generalize everything into black and white.
PS: They aren't suing their customers, they are suing a company that ruins the experience of their customers.