Or at least always be prepared to get the rug pulled. I also have a hard time to understand why anyone would use their free time and energy to help a billion dollar company to get richer for no reward.
Again, it isn't "providing unpaid labor to AWS". It's "problem solving and exhibiting talents to others that are in your field". It's showing you have the skill to throw such a thing together quickly. It's a resume builder. And most importantly, what you seem to be missing, it's a fun challenge to some people.
If I stretched your analogy a bit, I would say it's like seeing a body shop do a shitty job on a car, and then buying $30 worth of tools and doing a better job in an hour than they did, then posting the video to Youtube to show all your fellow car enthusiasts.