I think blizzard knows who bots and who do not do via warden. But I do not think that it is a problem for blizzard as long as you pay each month.
But when other players start to notice it and report you, then it becomes a problem and blizzard should do something about it. It's like with the police it's not all they are doing something about unless they get a notification.
They know that if you open a new account with them, so they still earn money from you and the other players will not know that its you, so therefore they have chosen to let people open a new account. If they really hated botters personally, they would ban you, your credit card and computer forever.