It is a business and it has to make money. However it is a pretty generous business because if you are any good at poker at all you will never have to buy chips and can play as much as you want completely for free. However you can’t really expect RP to allow you to help other people to play for free.
There is a kind of inherent contradiction. You can buy chips, but chips actually have no cash-in value, so they are not like chips in a casino not represent real money. When you buy chips you are just buying tokens that allow you to play a certain number of games. But if you win the games you never have to buy chips again, so there is that. Most people who buy chips to play poker in a real casino keep on coming back and buying more chips!