Maybe they don't think injecting referral links is bad behaviour. It doesn't hurt anyone's experience, and often gives a financial bonus to both parties in exchange for creating a database record that doesn't tell anyone anything they didn't already know from the User-Agent string or fingerprinting.

"The content you see on a site may not be the content being sent" is a low-effort excuse, too: plenty of users don't want to see plenty of things that site owners want them to see, and use browsers that display what they want to see.