It's been alleged that Apple threatened to withhold their typical promotional activities from labels who continued to offer exclusives to Amazon. Those songs would still see sales in iTunes, of course, but significantly less without Apple's promotional efforts behind them.
Back in March, Billboard reported that Apple's iTunes team had tried to talk publishers out of participating in Amazon's low-priced, exclusive "Daily Deals" promotions, which often motivated labels to offer Amazon first dibs on major releases at a significant discount. This allegedly prompted Apple to threaten to pull advertising if participation continued. Several labels supposedly relented and abandoned the Amazon deal as a result.
You'll remember that the DoJ also recently began an inquiry into a clause in the iPhone OS 4 SDK that bans the porting of software originally written for Adobe's Flash, Sun's Java or Microsoft's Silverlight/Mono to the iPhone OS. What a mess!
[Hat tip to Ars & Electronista]