It's actually logical - if after the demo you decide that you don't want the product you just don't buy.
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.
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? :)