Great question. To date I've written two negative reviews: First Date and Rocky.
Thing is, I pay for every cast recording out of pocket... So, if there's a score or recording out there I know isn't going to be great, I typically don't shell out the cash for it. And since I'm such an avid supporter of new musical theatre, I'm more interested in telling people what they should check out, not what they should avoid.
It's not too difficult to get cast recordings for free if you agree to review them all, especially since only a handful of labels tend to release them. Just contact the publicity contacts for the labels, and your mailbox will be flooded. I used to get free books all the time, as well as often get additional copies (sometimes signed) to give away for contests, etc.
If you're going to give such detailed reviews, why also pay for them?!