Maybe they say this because it is politically incorrect to state the obvious such as "I want my kid educated in English, not Spanish. I want them educated up to their ability not dragged down by the lowest common denominator." My sister lives in Texas and tells me legal immigrants feel exactly the same way, although they may speak Spanish at home.
There is a lot of money for the adminstration in bilingual education. I wish I could expound on it but I'm up way past my bedtime and not that coherent :o) People who are here legally, esp. the naturalized citizens, pay a lot of money to be here. They want the best. While they may be working three jobs eighteen hours a day, they went their children to be doctors and lawyers and not dishwashers and maids. An educated Mexican child can't be shoved into a special ed class.