We Americans can certainly, should we choose to do so, reduce or even eliminate the entry of South Indians or any other nationality/ethnicity. That is not an American peculiarity, but that of any nation to protect its borders as it sees fit. It goes back as far as the concept of the Westphalian state, and really much further than that.
That some Americans deny or have forgotten that we have this right—no, obligation—does not mean that it does not exist.
If there is a real problem with caste-based discrimination among and between South Asians in the US, I am pretty sure (at least, I hope) that it is not being committed by the American-born among them.
>Free speech rights granted by the US constitution doesn't care if you are a citizen or not.
I am not talking about censorship.