This is exactly what Flattr is all about (they're still around). Flattr has to deal with credit card fraud though, and all the issues surrounding it. This mean

This is exactly what Flattr is all about (they're still around). Flattr has to deal with credit card fraud though, and all the issues surrounding it. This means they need to spend a non-insignificant portion of their operational budget dealing with risk analysis for all of the payouts they make. And, as their userbase grows, the cost of dealing with fraud also grows. It also means they have to ask their users tons of invasive questions. Changetip, on the other hand, can accept deposits and make withdrawals with 100% confidence that they will not need to eat the cost at some point in the next 6 months. The downside is dealing with the security cost of storing their stash of bitcoins - but this is a fixed cost that doesn't grow as their userbase scales.