Building a trial version was not a first priority before launching. It certainly is something I'll look into next.
See number 7 in http://www.joelonsoftware.com/articles/customerservice.html
A direct quote: "You can try Radio Silence risk-free for 30 days. If you're not absolutely happy, you'll get your money back, no questions asked."
I'm not sure there's an entirely rational reason for it, but I don't think I'm alone there.
Incidentally, what happens if I ask for a refund, then use it block itself so it can't phone home for the revocation? :)
If after buying the product you want a refund you still need to contact the seller (or go through some automated process). It's an extra step.
As a seller though, I would think less people would ask for a refund than the group of people that would have bought the product, but instead downloaded the demo and then decided not buy.