That’s to avoid duplicate content penalty in Google and other search engines.

Normally the link even contains rel="canonical". Yet, the post wasn’t posted from theSteemObserver. The post was crossposted and posted through Busy as can be seen in the app data for the post.

No double standards. Only justified disclosure. Some posts on theSteemObserver were first posted to the Steem blockchain and that is correctly disclosed and set as canonical link in those cases too. If you tweet any of those posts from SO, they will redirect to the Steemit version of the post.