Jason Newsted got mistreated by that band from day one. He went on to play with Ozzy Osbourne and Voivod, which are great bands (the latter put out a great album while Metallica put out the utter dung that is St. Anger).
He put up with their bullcrap for over a decade just out of love for the music. He doesn't have a family, music is all he does. Eventually he got fed up with all the crap and left so he could be more creative, it's not like they let him write much anyway. If a band mixed my bass tracks out of their album completely, I would walk, no matter who they were. If a band treated me the way they treated Newsted on tour, I would walk. If anyone in a band that I had joined as a full member tried to order me to play a certain thing, I would walk. If a band changed their entire sound and look just to be more marketable or fashionable, I would walk. If a band only let me write a couple songs over a ten year period, I would walk. If a band flat out said that I was prohibited from having musical side projects when the rest of the band was taking a break, I would walk. No matter what band it was. Jason demonstrated incredibly admirable patience in sticking with them for so long despite being so under-appreciated.
I saw Newsted live with Voivod. He's a much greater musician than you or I will ever be. He really gives his all for the music, and he's worthy of some respect. My second favorite Metallica bassist.
Perhaps St. Anger was an album that Metallica needed to make to figure out where they were or whatever. However, it's not an album I need to listen to again. Thankfully, their newest release is much better, it sounds like they're actually working on that record. For me, there is no excuse for musicians of their caliber (except maybe Lars) to release an album like St. Anger. You might say they were going for a minimalist approach, but it didn't work. It's embarrassing, as a fan of music in general. That said, they have redeemed themselves a bit in my eyes, though I wish the sound on the new album was a bit better and that Rob's bass was a bit louder. Oh well.