Contractually agreed terms that state the game will be supplied on it's release date, with a priviso that the release date is beyond the suppliers control. As long as they deliver on, or around the release date as agreed, they're not in breach of contract.
As I said, it's a question of the T&C's, particularly the section relating to out of stock items. AFAIK, there is a legal provision to complete a contract of sale within 28 days for mail order items but that this priviso only relates after payment has been made, which is of course why you used to see all those mail order adverts in the paper that goods would be shipped within 28 days.