I don't know if the big sites should "block" those pages from being indexed, but it does go to show how far google's preference for brands has gone.
Many of those search results pages have VERY little content related to the keywords. Generally, that is due to the search functionality of those sites being very wide. They tend to error on the side of returning as many products as possible, whether relative to the search terms or not.
The companies should block them (I sure do but a lot of people don't) and Google sure shouldn't be showing them. Hopefully they'll deal with that at some point, I'm reasonably sure Google doesn't want them there.