This is a space that my company operates in and we will hopefully provide this service in the future. First, publishers want more than what consumers are willing to pay per article, the credit card companies charge too much (even with special micro pricing we've negotiated) and no one has a market big enough to reasonably get $xx per month into a wallet from a massive audience. Google had tried something similar with Google Contributor but has shut it down. There was also Scroll that recently got bought by Twitter that did a wallet-type of deal though it was more complicated than a flat rate per article.