They aren't making a market (like an App store), promoting my files, creating my content, or otherwise providing value that scales with the growth of the success of my product. Why are they entitled to $5 on my $100 or $10 on my $200 if that's the price I choose? Charging a fixed price per sale is more fair + any %fees you might incur from payment processing.
Clearly many businesses are based on the same % transaction fee, but it's not really fair if you look closely.