Perhaps we watch them because we are hoping that since the source material is often so good that the adaptation will be worthy of our time. Perhaps we watch them because the good ones like Shawshank and Stand By Me show us that making adaptations of King's works can be done correctly.
I don't see what the problem is with legitimate criticism. I criticize King's source material (and yes, I read every one that comes out) so why wouldn't I criticize adaptations of it. That I read it or watched it doesn't automatically make it good.