Simple math! Comcast loses money by requiring customers to stream instead of download in the app. I'm paying AT&T for data charges rather than Comcast for premium movies. Why!?
streaming your linear and on-demand and recorded assets in the home is not charged to your data Cap. Data used away from home for your TVgo, on-demand(available out of home assets) and recordings is not charged by Comcast unless perhaps you are using Comcast mobile service (don't know their charges).
Comcast has to make deals with the various networks as to what is available for download. Many networks won't allow it due to the fact it makes it easier to pirate (once they have a full copy, they could break the encryption.)