For any goods release (music, DVD, games, book, whatever) there's an embargo*, which prevents them from selling earlier than the release date. Some stores do try and break the embargo, but if they're caught they are made to pay a fine, and could risk future deals with suppliers. Generally the shops which break embargos are the small independent shops who need the sales and think they can slip under the radar, big chains are much more cautious. There have been cases of big chains accidentally releasing something early due to a staff mistakenly pulling them out of storage early or an admin error, but they're pretty rare. Knowing someone who works at the shop sometimes helps, but since receipts keep a record of the exact time of sales many stores would be reluctant to even allow staff to buy early. One of my friends used to work in Woolworths, and his boss would let him take a game home early and then run the sale through on the day of release to get round that, but it takes a pretty understanding boss.
*Not sure if that's the correct legal term for the policy, but it's what my boss always used to call it when I worked in WH Smith.