My first thought is that the green card sorta contradicts the illegal part of the idea.
As for the underlying issue, discrimination is the discerning between multiple things, in this case, legal citizens versus illegal entities :) from outside established borders.
To treat these two based on their differences, especially that specific difference, is the fundamental function of law. We are supposed to discriminate between those that obey the law, and those that break the law. Clearly we treat people differently based directly on said discrimination, and we do so with intent.
I would answer no, a costume cannot discriminate between anything. But I would answer yes to what I think is the spirit of your question, that it points out the differences (that which to discriminate is to do) between legal and illegal residents. Though I wouldn’t say it actively applies the discrimination to any decision.
As to the comment about all Americans being illegal immigrants, it’s false. There were no laws established that forbid us from immigrating to this land… hence the word ‘illegal’. I would agree that all of our ancestors (save native Americans of course) were aliens by that definition of the term, but not illegal.
If you are referring to immigration within modern day US borders into lands ‘owned’ by native Americans (though American indians had no concept of owning land at the time), we most certainly invaded them. However, that was not the ancestors of all Americans, far from. And if we admit to the fact that we invaded their lands, we must also admit to the fact that illegal immigrants are invading ours.
Many of our ancestors invaded their lands by picking a nice stretch and building a house without mass numbers, armies, war, or other.
This string’s question has moved me to ask one concerning discrimination as also.