It’s probably due to some licensing verbiage because these DRM implementations do just about nothing good. Pirates still rip videos from these platforms somehow, but the usual customer can’t watch the 4K they paid for on most browsers. So, it must be an idiosyncrasy at some management/licensing level, not something that serves a functional purpose.

And frankly, the “buy 4K, but we will serve you 1080p in most cases” sounds like a class action lawsuit in the making anyway. So, if anything, this DRM scheme will cost them more than just wasted engineer time.