Teaching only in English is not discrimination. It is a necessary form of assimilation. Students can, in general, not succeed in the United States without being pretty fluent in English. Therefore, whatever sort of education gets them to fluency (or at least competency) most quickly is the thing that is best for them.
That said, there is a great deal of conflict on this issue. Scholars are not able to agree on whether bilingual education is or is not more effective than immersion in English. This can be seen quite clearly in this link. My experience makes me feel that bilingual education is less effective than immersion. I feel that bilingual education often "babies" kids along instead of throwing them in and forcing them to learn English as quickly as possible. I have also seen it, I think, create a sense of tension between ELL kids and mainstream kids. This is not helpful at all.
So, I am not an expert on this, but even the experts can't agree. My experience tells me that bilingual education is not great and that immersion in English is much more of a tool for assimilation than a form of discrimination.