Such custodians as these choose to escrow USD and issue the stable coins onto different blockchains. They are always able to be redeemed for for their underlying assets via collateralization. However, there are some risks. One being, it is not convenient for users to confirm that a centralized authority has enough of that currency to reflect the collateral that has been reported. Another, is that such fiat-collateralized stable coins are subject to the risk of ever-changing regulatory institutions. It is for these reasons we believe such coins are not suitable for an truly decentralized culture.