Remember what lead up to the Alamo and the transfer of land from Mexico to the US? They do not want to repeat that. As I understand it, foreigners are not allowed to own land in Mexico within a short distance of the US/Mexico border. I don't believe the prohibition is a total prohibition against foreigners buying land.

We have that too but employers choose to break the law. Japan also does not allow foreigners to be employed, but some Americans who go there to study also teach English on the side for income.

I read some of the article you mentioned and it does appear that the Mexican constitution is anti-immigrant. I guess they need to clean their own house before they complain about US laws.