The question this sort of thing leaves me with is "Why does {Twitter/Facebook/Google} permit this? What are they getting out of it?"
Its pretty clear what the bot guys get out of it, pay to promote services, pay for followers, etc. They can monetize "fame" through the robotic horde. But as this article and ones before it point out, these networks are generally quite easy to spot. So why not take them out?
It probably isn't because they can pad 'subscriber growth' or 'MAU' numbers, they appear to be only small components of that number. And while I could imagine it may be hard to purge them at the moment, its been a problem long enough that someone in engineering must have figured out a system for taking down large numbers of accounts.
The only thing I can come up with, and it is way too tin-hattish to really count, is that it creates an "observable" for the underside of the Internet. By watching what people are asking the twitter bots to do you can observe other objectives that are perhaps less observable. There are some obvious customers for that but I don't think they actually pay for that (except perhaps by buying access to the Firehose)