The Roman Catholic Church, both in the United States and in Mexico, is led by the Pope. The current Pope is Pope Benedict XVI.
The Roman Catholic Church defines its mission as spreading the gospel of Jesus Christ, administering the sacraments, and exercising charity.
Individual countries, regions, or major cities are served by local particular churches known as dioceses or eparchies, each overseen by a Catholic bishop. Each diocese is united with one of the worldwide "sui iuris" particular churches, such as the Latin Church, or one of the many Eastern Catholic Churches. As of 2008, the Catholic Church (both East and West) comprised 2,795 dioceses. The bishops in a particular country or region are often organized into an episcopal conference, which aids in maintaining a uniform style of worship and co-ordination of social justice programs within the areas served by member bishops.
Dioceses are further divided into numerous individual communities called parishes, each staffed by one or more priests, deacons, and/or lay ecclesial ministers. Parishes are responsible for the day to day celebration of the sacraments and pastoral care of the Catholic laity.
Now, as you've observed, any particular Roman Catholic Church parish will emphasize one of the three missions. Either the congregation will follow the lead of the local Priest, or the local Priest will follow the lead of the congregation. In the United States, in general, congregations lead. In Mexico, in general, Priests lead.
Edited to add: What I mean by lead in the previous paragraph is that United States Catholic congregants will quit going to church if the Priest tries to stray too far from what the congregation wants. Unless there's only one Catholic church in town, congregants have choices. In Mexico, because of the culture, it's not as easy to change churches. Therefore, the Priest is given more latitude by the congregants.
Again, as you observed, the Priests in Mexico thought differently about exercising charity than the Priests in the United States. Part of the reason is culture, and part of the reason is who leads, as I mentioned in the previous 2 paragraphs.