"It's their money they can do what they want" is a bad argument. I can buy a sandwich in front of a homeless person begging for food and I can break it into pieces and feed it to the birds in front of him. It's my money I can do what I want. It doesn't change that fact it would be a disgusting thing to do to someone.

It's generally accepted that if you work with people and they contribute to your success you should recognise that and reward them rather than attempting to exploit their passion as far as possible. Elon Musk absolutely doesn't have to do that, but that doesn't mean people can't judge him for being an asshole.