This, of course, is referring to the use of "illegal" or "illegals" to denote people. It is always used pejoratively and becomes an excuse to hurl all sorts of slurs. If you as an individual are defined as illegal, as opposed to your immigration status, then it becomes easy to label you as dirty, criminal, freeloading, and worthless.
I tend to see "illegal immigrant" as much less pejorative, particularly how it is commonly used (though I know many disagree). That is why "undocumented immigrant" is more common for those who want a less loaded term, though even that is problematic given that many immigrants are documented in some way even if they don't have legal status.
Regardless, language matters in public policy debates, so this is not merely a matter of semantics. Language puts particularly types of images in people's minds. Whether that image is positive, negative, or neutral affects public opinion.