Lots of those are manufactured by the same companies that make the branded ones. So they can still get their cut.
I do wonder if Amazon are making any kind of profit on these items. With their long term strategy they can very well afford to make a loss just to undercut the competition and put their products out there, make a name.
Unfortunately, it does have a depressing effect on new design and innovation. And it certainly seems that the low-cost Chinese manufacturers haven't got the knack for originality yet.