The don't actually delete your posts, they mark them as deleted. If the government requests your data they get to see everything, even those posts you decided to delete.

I think the above post was referring to Facebook losing their data, by their database actually failing. That would mean the hidden deleted posts are also removed. Of course, they wouldn't have a single database or copy of the data, so this is irrelevant.

As for not deleting data that you request to be deleted, that's normal practice. I know on every site I run, when someone clicks delete, it flags the content and it's hidden, but remains in the database. Deleted data is still valuable data, and it has a wide variety of uses.