Here are a few points to consider about Spanish in the United States.

1)  Many people who grow up learning Spanish in the home receive all of their formal schooling in English.  This means that they do not have the opportunity to develop literacy skills in Spanish.  If their parents have low levels of formal schooling, there is even less chance that U.S. Latinos will develop an 'educated' variety of Spanish.

2)  Different communities express things in different ways.  An "autobus" in Mexico is a "guagua" in Puerto Rico.  This explains some of the variation in terms found across the country.  But how can gas station owners render a phrase like "Please pay before you pump" in a form comprehensible to the Mexican, Puerto Rican, and Central American customers in their communities?  Perhaps with "Por favor pague antes de pompar."

3)  According to the linguists Ricardo Otheguy and Ofelia García, when Latino communities confront a U.S. referent that is new to their experience, they often borrow the English term.  For example, an "edificio" in Latin America refers to a "small, well-kept structure that inspires admiration and respect," while a "bilding" is a "large structure that inspires repugnance and fear" (Otheguy & García 1993:147).  That is, speakers import English words in order to express a concept that is new to them and hence culturally adapt to the U.S.

Points 3 and 4 suggest that the "correct" word used in another country [Fill in the blank: Mexico, Spain, Colombia, Chile, etc.] is not necessarity the "correct" one for a particular U.S. community.  For example, Spaniards might say "abrir el grifo" to refer to pumping gas, but would anyone in Los Angeles, New York, or Chicago understand that?  And in places with quite heterogeneous Spanish-speaking populations, such as Chicago, New York, and Miami, should any one dialect prevail over the others?

4)  Some researchers consider that although U.S. Latinos may switch into English while speaking Spanish or use phrases borrowed from English, they are exhibiting a tenacios dedication to Spanish.  Despite the huge pressure to abandon Spanish completely and use only English, they continue to use Spanish, which should be applauded and encouraged.  Those of us involved in educative missions should validate what our students speak while encouraging them to learn synonymous expressions that are understood by a wider Spanish-speaking audience.
 

References:

Otheguy, Ricardo & García, Ofelia.  1993.  Convergent conceptualizations as predictors of degree of contact in U.S. Spanish.  In Ana Roca & John Lipski (Eds.), Spanish in the United States: Linguistic Contact and Diversity, 135-154.  New York: Mouton de Gruyter.