I think an app store would be a great location to not have optional payments for open source software if developers do not want it. Users can choose to go get and compile the source, download a third-party binary and keep it updated themselves, or pay for it and know it's directly from the authors and automatically updated and getting a path to support and the interested customers know the only people submitting reviews are people who paid for it and not drive by opinionated users like is common in open source software.