those are mostly cropped official promotional photos, like you see in pretty much all galleries for series. even behind the scenes photos are fine: https://www.themoviedb.org/bible/image/59f758339251416e71000065 (avatar 2 has pictures of the crew.)

why not let the users decide which they prefer?

Yes, but those are episodic images.

From the bible page you mentioned:

Television backdrops are added at two different levels: series and episodic.

Series backdrops are mostly used for group and individual promotional pictures. Some stills might be allowed as series backdrops if they are a good representation of the series.

Episodic stills are allowed when there are no promo pictures and the stills make good series backdrops.

There are promo pictures, and not all of them make good series backdrops. Someone just bulk-uploaded everything that was released. Needless to say, people tend to upvote garbage just for the sake of it, or for trolling and mockery.

Anyway, I'll leave it for the mods to decide.