In fact, Blizzard makes a lot of money from this.
They hold mass bannings, ban accounts, not IP addresses, so the botters make new accounts and pay for their accounts, so Blizzard makes big bucks.
World of Warcraft does have a two (or three) week try out period.
I heard it's possible to get an account up to level 80 in that time, mining gold along the way and selling that, and finally selling the account for big bucks!

Anyhow, these are all rumors I heard, don't take 'em from me. I don't even play World of Warcraft!