Bots have been farming for years.. blizzard is doing a lot more than people give them credit for. Like numerous people have said in this thread the only way to catch a bot is by the GM's actually observing them. There are millions of subscriptions in WoW and the ratio of GM:Player is staggering in favor of players. They can only do so much.
The best we can all do is report a bot when and if we see one.