Part of the agreement in the developers skills agreement is you not charge for the Skill.
A skill doesn't directly make Amazon money, true, but if it gets someone to buy something from Amazon then Amazon has made something from that no mater how small. Your Alexa app or alexa.amazon.com advertises different skills every week they then charge the developer data transfer fees that they helped to feed so they are making indirect money from skills even if it is just pennies.
No wonder good programmers and developers are hard to find.
They are getting nickel and dimmed to death.
If I'm going to make something and give it away for free I don't expect to be charged anything for giving it away.
Amazon does offer a T shirt for a submission (this month it is a hoody) If someone were to ask "where did you get that I want one?" and I say Amazon Guess what my skill just got Amazon a T shirt sale too