I think that any business on the level that Walmart is on in terms of volume and wealth generated is going to have its share of ethical challenges. Certainly, Walmart has had their level of ethical challenges. Walmart has responded to such claims in a manner which can say that progress has been made and that the ethical challenge has been addressed. Yet, these situations have not entirely dissipated and still linger around the perception of Walmart.
The most sustainable ethical problem Walmart faces is one in the realm of business practices. The idea of "predatory pricing" as one intended to drive out competition can be seen as ethically questionable. Walmart claims that this is not as much predatory, as much as it is a desire to deliver the lowest possible costs to consumers. The result, deliberate or not, is to drive out local and smaller businesses that cannot afford to price at the level Walmart can due to its bulk purchasing power.
Another ethical challenge Walmart faces is in the realm of labor relations. Operating without unions, Walmart sets its own compensation scale as well as its own employee practices. There have been some charges of whether or not some of these practices have been ethical, such as the denial of collective bargaining and addressing hiring practices.
Specific studies have shown that Walmart's hiring practices might not be ethically sound, especially in regards to gender bias. The questionable interpretation of child labor statutes is another area where ethical concerns have been raised.
In each of these cases, Walmart commenced an internal review and claimed to have steps to remedy the challenges. However, in the end, all of these add up to create some level of doubt about the sincerity of Walmart in the issue of ethical challenges, striving to put forth its economic viability as compelling and attractive to its brand.