[ursablanco]

Why should users feel so upset about a free app they downloaded that they would take the time to write a negative review, a review they probably didn't read before downloading themselves?

My guess - because, when a user goes through the process of evaluating a for-pay app and then deciding that they want to pay for it, they're saving face by not critiquing the app if they decide that it's not what they wanted. If they evaluated it, paid for it, and realized that it was a mistake, they're basically saying "Oops. I'm an idiot and I just wasted five bucks."