Doctor M said:

Why must they do this?

Taking a completed 2K file and creating a 1080p file, they can:

1. Crop only horizontally.
Maintains: 1:1 pixels, vertical image info.
Alters: aspect ratio by 6%, 128 pixel columns lost.

2. Crop horizontally and vertically.
Maintains: 1:1 pixels, aspect ratio.
Alters: 128 pixel columns and 54 rows lost.

3. Downsize by 15/16, generating artifacts that are especially visible for something with pin-point pixels (ex: ringing) The entire image is negatively altered.
Maintains: aspect ratio, horizontal & vertical image info.
Alters: all pixels malformed.

As nickdiba noted, #2 is common practice for Blu-ray releases, though I think "always" is overstating things. And overcropping for the sake of maintaining AR goes back to the LaserDisc days.