The odd part of that is that it's so inconsistent. There was an episode of South Park recently where the subtitles were useless. A line of dialog would be on screen long enough to only read 2 words, and then gone. Some shows have very inaccurate subtitles where the entire meaning of the dialog is lost.
Yet, some shows like the documentary I have on now have excellent subtitles (although still yellow. Who decided yellow with no background was a good idea?). This show's subtitles even point out exactly who is talking, and aren't overlapping any of the open captions (two guys on the show have a thick accent, so they're open captioned).
If they can get some of it right, why not all of it?