They didn't do it like for free. That is the point of stable coins, lets say you have 100 million dollars, you put it in a bank and than show that as proof of your 100 million stable coin has a backing from the money you put into the bank.

Let's assume people believe you and started to give you bitcoins for the stablecoin you have, now you have 100 million in your bank account that is proof of stable coins + you get bitcoins, lets assume everyone trusted you even more and started to give you more and more.

Eventually you will have 100 million dollars in your bank account and have 100 million dollars worth of crypto currency as well. With those currencies you could basically sell all of them at once and withdraw it to cash. Now, you have 100 million dollars in your bank account and another 100 million in another account. You basically made 100 million dollars for free. Its a no brainer for people who can actually do it.