Really, the fact that they switched to serving the images from google.com instead of recaptcha.net shows it's just another avenue for them to collect google auth cookies.
@OP while I completely agree torproject should not use google for captchas (wtf!), just FYI tor browser has a different set 3rd party cookies for each site so even if you're logged into google while getting a google captcha on this site you will not be sending your logged-in google cookies with the captcha requests.