It's really a fine balancing act of trying to deliver the best customer experience (Which Amazon really does want to do). Without the co-operation of publishers, there wouldn't be any content, and then what's the point? Before Amazon pushed the publishers, the eBook market was tiny and unfriendly. Getting the experience as good as it is today has been a colossal amount of effort.
I don't know how enforcement is on scammy books, but I can't seem them being a positive user experience, so I can only assume that some action should be taken.
But at the same time, you really don't want to take down stuff unless you're sure you want to. It is non-trivial to determine who owns what content, and requires human intervention.
So buy a Nook. Or a Sony.
I have a Nook which I use exclusively for out-of-copyright content. (I doubt I'd ever pay real money for an eBook, since I like paper books far too much.)
Yet they totally leave the door open for scammers to upload content they don't own the copyright to and have no interest in really enforcing that side of the business.
It's the main reason I won't buy a Kindle, as much as the thought of owning a beautiful ebook device reader attracts me so much.