The West Wing was shot on film and six of the seven seasons were broadcast in high definition. Warner Brothers made all seasons available in high definition through download and streaming, but the plot recap and opening title sequence are in standard definition. Why?
This is just a possible guess, but it's likely it has something to do with the process needed to create the plot recap and the title sequences. I imagine the cost-analysis showed that it wasn't worth it to recreate those from the original film source (they'd have to re-edit, re-cut, re-mix the audio, etc.). They also didn't do anything with the audio when releasing the HD downloads, leaving them stereo only. Saving a bit of money (and bandwidth because your resulting digital files are now a little smaller) on things that people probably don't care much about when watching a show online can add up in the long run, especially knowing that a measurable selection of the viewing population would be using laptops, mobile phones, tablets, etc. where the difference between 480p and 720p (which at least Amazon Instant prime is ... don't know about iTunes) is not very noticeable.
If they ever release a Blu-ray, Warner Brothers might make different choices.