My guess is that they fear vendor responses would turn the review system into a noisy discussion/support forum. This would be distracting, possibly intimidating

My guess is that they fear vendor responses would turn the review system into a noisy discussion/support forum. This would be distracting, possibly intimidating, and require costly moderation. These are reasonable concerns. Keep in mind that the purpose of the review system is to make the exceptional apps float to the top. The rest of the apps don't matter much to the store. In Apple's store, you can provide plenty of text and a support link. And you get a fresh rating when you update. When I write a review, I am typically not looking for a response. When I am, I contact the developer directly.