I think you will find that trail building there isn’t sanctioned so much as tolerated . the signs will be there to emphasize a need for it to be curtailed/limited for a while. Or just to remind people that they don’t want any activity. Many a time have I seen people treating no riding as an excuse to do something else there instead.
Slightly bemused by the idea of building those tracks. Dowies was an evolution of a good line picked, and rubbish moved to make it rideable.
Sadly of late the area has been attacked by those with spades but no common sense. Many bikes with spades on board come past my place on the way back from the Dowies area.
Last point. Our views don’t matter. We do not own those woods and if the FC wish to make daft and contradictory statements they can do so. Not our place to decide if we like some rules and not others.
Having said all that, as I do not regard the FC as fit owners of the Forest, I ignore the signs. However I don’t make a deliberate attempt to turn the place into a detonated mine field. Leave the tools at home and take it for what it is.