Huh, you're correct. I had this memory of having funded someone via Paypal, even could visualize the Paypal email confirmation, and I go and look, and it is indeed an Amazon Payments confirmation instead. Weird lapse of memory; sorry for the noise!
In fact, it is specifically due to Amazon Payments simple mechanism for getting a pre-authorized token that can later be captured that allows Kickstarter to run that business
(PayPal now supports this through PayPal X, but did not at the time that Kickstarter was being first released. Getting access to this API from PayPal requires more authorization than from Amazon, but the result is also more powerful: you can hold tokens with no expiration and seemingly no cap.)
Looking through the AdaptivePayments API, the limits on preapproval are one year and $2000, although you can ask to raise those limits on a case-by-case basis.
