The sign-in page requires me to answer a captcha, while I can get it on some pages, in services like this, particularly when clicking on SSO buttons this is both ridiculous and annoying.
This is added to the fact that the session expires randomly, and the captcha adds to the annoyance.
Leave the bot filtering logic to the 3rd parties, Github and BitBucket, that is why we use SSO in the first place (well, some of the reasons why, but you get the point)