[missingcolours]

In those cases, the company controls all of the code running on those sites, so it's desirable for them to share data and cookies in particular. (e.g. any google.com site can read your login cookie)

In the case of user data domains, intentionally in the design of the service or via a security hole, users may be able to execute code and read cookies (e.g. in JavaScript on a page hosted on githubusercontent.com) and that's undesirable.

[usr1106]

Sure, I see why as a company you don't want user data in your domain.

But if the different domain name gives good protection / isolation, why does Google still use completely different domains for different services with content controlled by them. I cannot believe they are interested in protecting users from data collection.

[plorkyeran]

YouTube was an acquisition that they didn’t rebrand. Google Video was on google.com. gmail.com redirects to mail.google.com, and only email addresses use the gmail domain to avoid appearing to be google employee emails.