I think I know why the employee made the accusation he did. The imposition of proof of citizenship requirements is widely perceived as having a racist purpose. I don't agree with that perception (I think it has a classist purpose), but lots of people do. Therefore, it's an understandable -- but logically faulty -- leap to thinking that all supporters of the idea are racist.

I think this is another example of something that I see all the time, both in everyday life and in the comments here, of assuming what someone's motivations and attitudes are based solely on political labels or expressions of support for/against particular political proposals. 

Mr. Kidd even does that himself here, by referring to "the left" collectively not only as if everyone on the left thinks the same way (they don't), but as if any single individual can somehow be representative of what "the left" is and thinks (they can't).