Yeah that they do it globally but not in the US (other than a kind of meagre payment transfer on FB that used to exist) makes me wonder if they were bullied, federally, into not doing so.
A simpler explanation is that US doesn't have the banking infrastructure to support these kinds of micropayments. ACH transfers take 3-5 days and require you to link a checking account. Debit & credit card transactions have high fees. Chargebacks/fraud are a constant problem. So no payment processor wants to take on this complexity and liability.
