Google collects a really large amount of information regarding your computer and you in order to help prevent bots from using your website. And, they try to tie that information back to your existing Google account.
I could see why someone wouldn't like that because it also inherently has to let Google know which third-party website you're on (E.g., imagine someone has an "adult" forum and now you have to tell Google that you're browsing it due to the CAPTCHA system).
Personally, I don't care enough to complain. I'm already spied on 24/7. But, I could totally absolutely understand why someone else would want better self-hosted CAPTCHA options.