There are lots of incentives and motives at play in a system like this. For just about every downside there's probably an upside, the question is which outweighs the other (and that may be relative to the person). For example, the person who reads a lot isn't just extracting content for a reduced cost, they are also (possibly) rating a lot of books providing more accurate market info. Their reading, if more eclectic due to volume, may provide more support for more independent authors, allowing for more variety to exist (and be rated). Now, that is a lot of maybes, but marketplaces live off information, so increasing market information is an important task, and this structure may be one way to help with that.
