As much as I am a full feet up guy, having been trained for it from the first live flight, that is not what Airbus says.
If they had a justified reason why UP was the desirable choice they would had said so. Again, the way a great many comply with the instruction is feet up with no dragging issues at all. Yet there must be a reason behind the relatively complicated, result focused wording. (which frankly is lawyer's talk and them washing hands - so they had decided not to provide an opinion - take note here).