I see this story come up a lot, but according to reCAPTCHA, it's an urban legend. There is not really any evidence that spammers actually do this at all, let alone do it on a meaningful scale.
Unless my memory is playing tricks on me, I recall Luis Von Ahn mentioning this as an example of ways people had attempted to defeat his system at a talk of his several years ago. He may have been talking about theoretical attacks and not actual ones, but I'm on my phone now and can't effectively dig for a video if one exists.