Walmart is very successful. What laws are they breaking? Should we put limits on success?
They aren't breaking any laws, but mere abstention from breaking laws doesn't necessarily mean they're acting in an ethical way, or in a way that helps America at large. The disagreement over wal-mart foregrounds a number of interesting issues: for example, by moving into a sector that has historically provided means for people to move into the middle-class (unionized grocery stores), wal-mart, assuming they're as successful in the grocer business as they are elsewhere, is going to shift a pretty big swath of people from the middle-class to the lower classes (since they'll make far less and have far fewer benefits).
Also, as a matter of public policy, should governments allow companies to move into their borders that effectively shift the responsibility and burden of providing health care from the company to the government?
Is saving money on groceries worth the attendant difficulties? It's a pretty interesting question.