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.
As a non-chrome user things have been getting a lot worse lately. I've also noticed more and more pages using recaptcha even there's no clear way for bots to spam content.
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.
