This article by the Federal Trade Commission appears to suggest their business practices are not lawful.
From the link:
requires sellers to have a reasonable basis for claiming they can ship an order within a certain time and details what sellers should do if there is a delay
By law, a seller should ship your order within the time stated in its ads or over the phone. If the seller doesn't promise a time, you can expect it to ship your order within 30 days.
The shipment "clock" begins when the seller receives a "properly completed order." That includes your name, address and payment (check, money order or authorization to charge an existing credit account — whether the account is charged at that time or not).
If the seller is unable to ship within the promised time, it must notify you, give a revised shipping date and give you the chance to cancel for a full refund or accept the new shipping date. The seller also must give you some way to exercise the cancellation option for free — for example, by supplying a prepaid reply card or staffing a toll-free telephone number.
If you don't respond — and the delay is 30 days or less — it's assumed that you accept the delay and are willing to wait for the merchandise.
If you don't respond — and the delay is more than 30 days — the order must be canceled by the 30th day of the delay period and a full refund issued promptly.
How to file a complaint.