I don't think there is any law against selling it earlier than the release date.
Unless of course the retailer has signed a contract to say they would not, in which case they would be in breach of contract.
Online retailers send the products out early so they are likely to arrive on the day of release. This helps them with the problem of people paying the little bit extra from the shops, because they want it on the day of release.
Anyway, I'm not bothered cause I got my copy last night from Big W, who had signs everywhere saying it was released today, but were quite happy to sell me one of the many copies which were on the shelves.