The term "illegal", as in "illegal practitioner" or "illegal alien" is aptly used to describe someone who does something illegally.  No one seriously suggests that the person himself, rather than his action, is "illegal."  Furthermore, "illegal alien" is the term used by the U.S. Code.   That fact probably explains why the term is used.  People shorten expressions or terms in informal speech, and therefore use the shortened term "illegal" as a noun rather than an adjective to modify the noun "alien."  Big deal.