If they do not guarantee any return on whatever investment they sell then they are under no obligation to look for their clients best interests.
It happened during the subprime mortgage crisis where banks would sell derivatives to their clients and short the same clients because they knew the products were bad. And it is totally legal.
So who is to blame ? the banks ? or the elected representatives that made these things legal ? in the end the people is , for electing people that serves bank interests.