Imagine if you'd limited Elon Musk's PayPal wealth with such confiscation taxes. We'd literally have no SpaceX or Tesla. Why should society put so much trust that politicians can allocate resources in a fair and economically beneficial manner?
As far as I'm aware, Musk can and does pay tax on his earnings
In fact you see some private contracts which are exactly like that NASA contract: "We want to pay you to create a much cheaper product than we can already buy, and here are some early progress payments so you don't have to finance it yourself."
You may think this is a moot point, that's totally fine. The difference between subsidies or not is important to others.
Yes, there are smarter people than him but how many of those people have Vision he has plus the ability to execute it?