Nestle is certainly not the only food corporation with problematic labor issues in its supply chain. By carrying out its own investigation and then being honest about the results, the company is setting a good example for its peers. They may not always have control over the labor practices used by companies they do business with, but it's important to take steps to discover unethical practices and then be open about how the company is responding to them.
In big businesses, publicity and keeping a high-quality reputation is a necessity. Nestle may very well be genuine when they claim they will start self-policing their supply chain, but I believe it will not be in the way that we think it should happen. Corruption is everywhere, and most people/businesses need an unbiased third party to help keep them honest.
If a corporation finds a loophole or any way to save money or make extra money, it will. At the same time, it has an image to protect, since having a good public image will help it to make more money as well. The best possible solution to an image issue is to publicly admit that it was doing something wrong, that it was a mistake, and then to continue doing whatever it was doing that hurt its image. Admitting the mistake will help its image, which will help profits. Clothing companies have done this over and over regarding sweat shop labor: Claim ignorance, vow to stop doing it to regain public favor, and then continue to do it.
I don't trust anything Nestle says they are going to do based on their past performance. When they discovered contaminated baby food in the United States they pulled it from the shelves, only to then sell it in Mexico. This company has been all about the money and if tainted things are coming through their supply chain they probably think that's okay if it's cheap.