For me personally I was very impressed with Scirra's conduct and the product, I was obligated to pay for the product because I felt they genuinely deserved the money, in fact when I thought the payment transfer may not have worked out properly I sent an e-mail to their support address and was rewarded with a flying monkey toy for my efforts, that's customer service!
Sales and impulse buys are another big push, people are much more likely to shrug and part with their money if they think the opportunity could pass them by.
With a legit copy your getting a solid product from a reliable source, a cracked .exe from a Russians computer packed with spyware and toolbars doesn't exactly entice me.
The truth is you won't stop everyone, you're relying on the fact that your customers are honest individuals, and most of the time they are.