You're a little incorrect there, peerpee. Amazon just tend to price match others, at least in the UK, and in the majority of situations they're not the cheapest for DVDs outright. They're usually just price matching one of thehut group's stores, Play.com or HMV, which means they're very rarely the cheapest after discount codes and cashback are taken into account. It's HMV who are employing the strategy of being super competitve on pre-orders and Amazon are just matching them.
The overarching point is that Amazon.co.uk sell far more product than any other etailer and, as you describe, they have the power to match anybody. Nobody else has that power and labels are frowned upon if they sell directly from their own sites cheaper than Amazon's price (once it's settled).
Ultimately, the labels have the power to charge Amazon what they want for their product, but there's a herd mentality about the whole industry which seems to keep everyone in line. I don't really understand it – but I do know it could be approached more elegantly with better rewards for the labels (who take all the risk and make the products).
HMV are in shitstreet, trying to play catchup after many years dawdling, and they probably have only until Xmas for it to work. They barely got through last Xmas and are being bankrolled by RBS and Lloyds, also now both in utter shitstreet. The Russian oligarch Alexander Mamut has been circling the band of wagons for a few years. They're being asset stripped whilst still conscious. [All this is online, it's clear what's going on.]