Example: If you rented "The Interview" on Google's video service (in HD) and played it back on a software/hardware stack that didn't have the latest DRM(1), you were limited to SD.

You paid for HD. You got SD.

Of course, you could go pirate a full HD copy of the movie and play it back anywhere. The effectiveness of these inconveniences is overwhelming.

(1) I'm not positive it was even possible to redeem an HD rental in HD. I don't know anyone that didn't get forced to play in SD.