Actually this isn't some scheme Comcast hatched. Comcast wasn't given a choice by the FCC. They were facing a federal mandate to stop analog broadcast over cable, they even managed to extend that deadline by three years. They were losing all of the air broadcast content anyway so going digital was simply inevitable. Comcast structured their deal so that they profit from it in the long run but even renting you all cable boxes for $2 a month doesn't do much to stem the cost of billions of dollars in fiber optic infrastructure for the changeover. You can bet they're still paying the cost of those improvements you're enjoying.
The infrastructure was already in place when Comcast built out their digital cable plant years earlier. The purpose of the digital migration was to reclaim the bandwidth used by the analog channels and use the much more efficient digital channels in their place. They needed this extra bandwidth for increasing HSI speeds and expanding VOD options. Their other option was to upgrade every plant with additional bandwidth capacity. It's estimated that buying the DTA's saved Comcast 90% over what upgrading the plant would have cost them