this is fiat finance business... some things are "legal" that are not moral
almost certainly they are betting on the demand for sell to be at some level on a statistical average and so they are not going to neccessarily maintain balances that *should* be there and at some point it can happen that more withdrawals are demanded than they actually have in UTXOs
that's why you don't use them, if you understand fiat finance, you want to save in bitcoin, you buy and hold bitcoin in keys you control physically