You're right, but I don't think there's any incentive to do it. They pride themselves on having no tracking or otherwise user information (not even app popularity statistics), and adding any form of payment would require managing user accounts or coming up with impractical multi-device proof-of-purchase sharing solutions.

It's also worth mentioning that since everything on the store would still remain free software, making it legal to share purchased copies, people seeking to use it as a business model (like the parent post) would generally be ill-served.