Off the top of my head, one reason could be that internally discussing grievances is more acceptable at one company, so there is less of a mental barrier to discussing it publicly. Or (and this is actually pretty likely, I think) maybe even just seeing people complain about Amazon online all day makes people more comfortable doing it themselves on Blind. I bet I could come up with more reasons if I thought a bit longer.

I'm not saying I think I'm right in this case but I also am not saying I think websites like blind/ratemyprofessor/etc accurately reflect how the general population actually feels