I fundamentally get what you're saying about buying a full game being very different from paying to develop something unfinished. However, I guess I just don't agree with the rest of what you say.
For one, it seems very clear to me from reading the store page that funding game development is exactly what anyone who spends money on this is doing. It says in many places the game in unfinished and under development. If anyone doesn't want to do that, that's perfectly understandable and reasonable, stick to the free demo. However, that doesn't mean there isn't anything to steal and the developers can't complain about people doing it.
This isn't a non-existent product, it's an unfinished product. Stealing blueprints is a crime, stealing ingredients is a crime, stealing prototypes is a crime. I just don't understand your reasoning there, it doesn't seem to match reality at all.
The service being offered for money here is a more developed version of the game and they're charging money for it. It's not for everyone, sure, but it is a legitimate business model.
And don't even get me started on the fact that people have paid for three wheels and a windshield on the floor expecting a whole car eventually, just ask Tesla.