How? Well, you kind of answered the question already! http://www.lukew.com/ff/entry.asp?1508

Amazon doesn't disclose details of the Prime program publicly but according to estimates Amazon Prime members:

Increase their purchases on Amazon from $400 a year to $900 a year after they join. (source) Spend 130% more than regular Amazon customers. (source) May be responsible for as much as 20% of Amazon's overall sales in the U.S. (source) 82% of Prime members buy on Amazon even if the item is less expensive somewhere else (source)