That is a "feature" of ReCAPTCHA, and I believe there might even be a patent on it. The idea is that if you are sure it's a bot and want to deny them access you still waste their resources by making them solve impossible captchas.
Are people really that eager to send google all traffic information? Is this really the best solution? Couldn't one for example parse the actual content that the bots are trying to create and filter/ban them based on that? I believe spam detection is quite effective these days.
This whole captcha hell is making web really annoying for normal users and mostly just benefits google.
It really fucks with you when you're on TOR, especially.
Looks like it happens not only in that case.