A bit of strange to me to see this lock withdraw option. One thing that comes to my mind with lock withdraw is that if someone get access to your account, a hacker maybe, then he cannot withdraw that money. But still that doesn't make sense with permanent withdraw locking.
Another use case for locking withdraw, that a gambler give his account to someone else to gamble on his behalf but he does not want that person to withdraw but i don't think this is the intention of Bitvest when setting up these new rules. Only LightLord can put some light on the reasons for these new rules.
Just before your comment, Light Lord has highlighted the 3,2 and 1. Amd you can go back and read what he said there. Many people were confused about the locking of funds in the casino but with explanation from the Op I think it is much clearer now.
As for @Dunamisx yes Bitvest has a good interface with nice games though I have not played but they have both old and new games in the casino.