Crowdfunding is an inherently speculative activity, a way to invest in a team with the hopes that they will create a product that you'd like to see in the world. Trying to fit Indiegogo into a presales model to justify PayPal's outrageous behavior here is a Procrustean approach that helps no one.
And who's trying to entice them? Again, if doing business with Indiegogo projects is incompatible with Paypal's risk profile they shouldn't do business with Indiegogo projects. That seems straightforward.
Bizarre to think we should be sympathizing with, enticing, or otherwise fellating PayPal in this situation.
While I understand how easy it is to sympathize with the merchant here, as none of us will ever be payment processors, the reality is that the sum of your suggestions or assertions, if explored just as a thought experiment, quickly lead to a world where nobody can do payment processing without imposing absurdly high fees. Of course we can all see how unfair it seems to have all this money promised by seemingly wide-eyed and clear-minded open source software supporters. But Paypal has millions of clients and has to act in a way that generalizes across all of them, and which requires the least amount of trust to be placed in those clients. The only argument that Mailpile has is "but we're not evil," which is true, but still insufficient.
...nothing is being sold.
Crowdfunding is only popular because it's got a high success rate though. If only 50% of projects succeeded and provided rewards, and 50% took your money and ran, how many supporters do you really think would be left?