I totally agree. I don't get why 37 signals would be at all interested in taking one time $10 payments from Moms and Pops when they have monthly recurring revenue coming in from a bunch of companies. Even if they have the infrastructure, just the support overhead from un-tech friendly folks will be a pain.
They already have the infrastructure for this and they have to maintain it running for Basecamp so what they're doing here is more of their "sell your byproducts" thing.