It's not the argument about whether melamine is poisoning or not but it is an ethical problem.
All Chinese gluten (wheat, rice and corn) for pet food around the global contains melamine. This makes people think that melamine is deliberately added in to boost the false impression that the protein content of the gluten is higher.
Because the level of the protein in gluten is measured indirectly from the concentration of nitrogen. The higher the concentration of nitrogen the more protein in the gluten.
Melamine has very high level of nitrogen therefore by mixing it with the gluten will make it appear to have more 'protein' in it.
Although this trick is the common practice in the commercial world, however, at least you have to test the safety for pets before doing it.