Not everyone has nefarious plans. Some people want to simplify the process for both parties. You have valid points, but there's no need to be so aggressive.
I'm not so certain, supporting saving data to a URL hash is pretty trivial to do, much easier than going with SSO integration - so I'm surprised the site doesn't support both at least.
URL hashing is annoying. It ends up transferring to someone else if you copy+share the link, and it disappears if you leave the site and come back later.