Business really has no ethics, and it needs no ethics: its main and only goal is to make money. Government’s job is to define the ethics, and create and enforce a framework in which businesses may operate.
The reason why businesses shouldn’t be responsible for acting ethically is that being unethical gives you an edge against your competition. So if we let companies have the main responsibility of how to behave, nice companies are penalized.
The framework needs to be as simple and unambiguous as possible, because the more complex it is, the more it penalizes small and starting companies.