A recent discussion (although this discussion pops up at least once a week) on the Amazon Writer's Cafe is about scammers in the Amazon book store. There are people that either throw together mediocre books, or use programs to create "books", or steal other people's books, then use click farms and other nefarious means to boost their book rankings up into the stratosphere. Then of course there's the issue with the keywords, where people intentionally abuse the keyword system to boost their rankings, but when everyone does it, all it does is make the search function useless. All of this combined results in lower payouts for legitimate Amazon authors.
Now, I'm just breaking into the industry myself right now, so seeing authors complaining that their meager paychecks are dwindling every month and Amazon doesn't seem to care one bit is daunting and demotivating. For whatever reason, it doesn't seem to be a problem Amazon is taking seriously yet, but if enough customers and legitimate authors complain about generic, bad, or fake books that are ranked highly, and a cluttered store where it is impossible to find the product you are searching for, I'm sure they will take action.
Here is an article that explains the issue very well: