Why do free trade agreements grant corporations the right of 'national treatment' abroad?

If General Motors invests in Mexico, it must be granted the rights of a Mexican business. If a Mexican of flesh and blood were to arrive in New York and demand "national treatment", he would be lucky if he did not end up in Guantanamo. The example is not frivolous. While corporations are LEGALLY persons under the law, with rights far beyond those of human beings, non-resident aliens are not persons, so the courts have determined.