Since when did Mexico legalize private gun ownership?
They didn't. But, when given a choice between kidnap or murder of you or your family, and illegally owning firearms, the logical and pragmatic choice is owning guns.
And, since owning those guns is illegal anyway, what you own makes no difference. It's all the the same, single shot or full auto. You just hope there are (1.) No witnesses, or (2.) Any officials involved will accept cash to look the other way, or (3.) You are very well connected politically, socially, or by marriage.