In the case of Terminator 2, the studio wanted to cram a lot of bonus features onto the same disc with the movie and program them to be "interactive" during playback of the movie, as opposed to putting the supplements on a separate disc. That, plus elaborate and gimmicky menus, took up a lot of space on the disc. So the authors turned up the DNR to wipe away film grain, thus pushing the bit rate of the movie down.

The older, non-Skynet Blu-ray edition uses the same video master but has less DNR. It's not great either (and only has lossy audio), but it's slightly less awful.

Compression encoders are really at the bottom of the Blu-ray production chain and do what they are told by higher ups. It got a lot of attention during the format war only because the formats had different specifications. WB and a couple of the other studios have largely automated the process, which is why almost of their encodes look similar in structure.