Mexican airliners, as do airliners of many countries do not fly to American airports as you falsely suggest. They are allowed to fly into a port of entry and then they depart the United States. So take your straw man and restuff him Mr. Logician.
I think you are.
If you go to the Mexicana Vacation Packages page, you will find that entering a US city as your departure point, only Mexican destinations will be available to choose from. The reverse in not true, as Mexicana is the principle airline in Mexico.
Whether one travels on Mexicana to Cancun or on a US carrier is a matter of personal choice. Would you deny them that choice?