Obscuring is about making it reasonably difficult to extract the wall image from the screenshot.
This naturally only applies to people whose screenshots contain walls that are not their own, or which they have no permission to use as such....Litestep themes, for example often have a third party wall included with consent...and since it's with consent means there is no issue with it in the screenshot either.
If any 'slip through' that is generally by accident / oversight ...when they are seen in moderation they are responded to....