They generally are not spun down because the box uses it to buffer TV content all the time. So you can "rewind live TV". The drives are not just spinning but actively reading, writing and seeking. They basically copied that from TiVo.

https://www.latimes.com/nation/la-na-power-hog-20140617-story.html

The figure before the HDDs was 18W average usage by a cable box. Then it went to 35W with HDDs. Now that HDDs are being removed the figures are dropping again.