That's a popular myth. Carlos Slim is the "prestenombre" for those companies. It's a different business structure, and he's just the head of a very, very, very larger partnership. The difference in Mexican and U.S. law is that a real, live breathing person is responsible for the actions of the business. In the U.S. a corporation is a "person" under the law, but if that "person" commits an egregious act, the only punishment "he" can receive is monetary... where in Mexico, the live, breathing person can go to the slammer.
The Slim family is no myth. Try to introduce a new broadcast station or telephone system in Mexico and see how far you get.