You may be correct, but that doesn't explain why Facebook decided to include so-called deleted files in a download of user data. Clearly these deleted files are still a part of Facebook user profiles and accessible to company data mining software. Facebook has exposed their own duplicity.
But I wouldn't discount your hypothesis.
Such an ethic creates a moral reasoning to not comply with an individual's wishes in the immediate deletion of data.
(FWIW I'm not defending this position nor suggesting it's the case here, just you said there's no moral reason that can support it, which seems wrong; different ethical systems can provide different reasoned moral outcomes.)
https://www.theguardian.com/technology/2018/mar/16/silicon-v...
Or possibly they just screwed up. Perhaps the "soft delete" was originally intended to allow "undelete" by the user with delayed purge, and/or single-instance storage with reference counting that they never quite got around to finishing.
This happened because the person tasked with writing the code to build the archive forgot to include the filter for "deleted" records somewhere in the code.
I.e., they forgot the "where is_deleted = false" part below on one or more DB query requests like this:
select * from table where is_deleted = false;
This is the biggest problem with the "soft delete flag in database" method of deletion. Every single query writer, everywhere, forever, must always remember to include the "is_deleted" filter in their queries. And when they don't, what was deleted reappears as if it had never been deleted at all.