I don’t think you have captured their take on immigration correctly. If it is only illegal immigration that is bad, that would mean that changing the law to make them legal would solve the problem. But that’s not what they want.
They don't say illegal immigrant is bad as a legal status (so that merely changing the law to make everyone legal would make it ok).
They say that illegal immigrants are bad for the same reason that the lawmakers made a distinction between legal and illegal immigrants: to place quota, immigration criteria, and so on.
