If you give something away for free, I'll take it for free.
If you give me the option to pay $0, and give me a reasonable suggested price(this book), I'll pay the suggested price.
If you give me the option to pay $0, show me what others are paying(humble bundle), I'll pay the median.
If you give me the option to pay $0, and give me "bonuses" at certain price breaks (kickstarter, humble bundle), I'll pay at the most attractive price break.
It actually makes a ton of sense to adopt these kind of pricing structures because people actually pay!