Scribd would rather increase the amount of content on it's servers by making uploading easy and free. While the guy that publishes the article is really the Scribd user, he can publish elsewhere if the price makes it too much of a hassle. The viewer has less of a choice, if it's only published on Scribd it's either pay up or go without.

I don't agree with their choice (or methods) but it makes sense for them to do this.