The laws in the US arose from Judeo-Christian moral principles. Ethics is an umbrella term for different systems of morality (i.e. relativism, judeo-christian, absolutism, etc.).
So ethics underly the law. The law enforces ethical behavior. That means that when a company says that it acts ethically, that can mean a whole host of things because, as said above, ethics is a catch-all term.
Well, I'm a ret. business consultant and I can tell you that a lot of companies do try to conduct business ethically, but it's NOT the majority. The foremost objective is to "increase the return on investment." That means to do whatever it takes to make money for the shareholders. That is the bottom line.
Sometimes that can be done ethically, sometimes ethics becomes a very fluid term because what may be ethical to you may not be ethical to me.
A good example is comparing how companies do business in different countries. Nike, for example, does not employ child labor in the US. They portray themselves as being an ethical company. They do employ child labor in Indonesia and hire Korean subcontractors to supervise the manufacturing of shoes, so when something goes wrong, they blame the Koreans. Now we cannot tell Nike what to do in Indonesia, but we can tell them what not to do in the US because in the US, we believe it's morally wrong to put children to work and we produced the laws to back it up. Now just because Nike follows the rules in the US does not make it an ethical company, does it?