I mentioned this idea to @abh12345 a month or two back. I think they will work on a limit of memos per account though. What would have to happen is many accounts each day would have to send 0.001 SBD or Steem to bots.

Now, here is the interesting thing. They generally group them so you may get 3 or 4 memos with a 0.001 transfer. Essentially, everyone who takes part in the drain will make 2-400% profit on each the transfer. It isn't a great deal but for some, potentially worth the effort to take part.

It is like sending junk mail with free-postage back to sender. They will still pay and if everyone does it, it will put them out of business incredibly fast.