Pointing out their flaws and where they got their material doesn't take the money out of their pocket, the music out of your ears, or (in this case) put the proper respect into the heart of the original creator, want to know why?
Most of them are either dead, dying or heading somewhere similar. Music is a recycling process of great efficiency. A good example is Fleet Foxes. They base a lot of their music on baroque acoustic jams, much like musicians in the dark ages did. Now as far as I know everyone in the dark ages is deceased, BUT they are still the original creators of the genre, and possibly some of the chord changes. Does that mean that people should shy away from the band, because they are 'stealing' from a past genre. Even though it dates back hundreds of years?
Its all a bunch of pointless meddling. People calling this better than that, or who deserves what. Its music, if you enjoy it great. But if that plagiarism factor is too much for you, don't listen?
After enough time passes, it doesn't matter anyways who created what or who deserves credit.