Warning: This is not a statement about the current immigration debate in the US. This is a statement about rhetoric and the use of language.
I have often heard people say “we shouldn’t use the term ‘illegal alien’ because there are no other instances where we label the person as ‘illegal’ rather than the action.” Not true. Two examples immediately come to mind: thief (“illegal taker of things”) and murderer (“illegal taker of another’s life”). And there are plenty of others I am sure you can come up with (“embezzler,” for example).