>the word "illegal" is used to refer to civil matters all the time (e.g. illegally parked car)
That's referring to a thing not a person. You would not label the owner of that illegally parked vehicle as an "illegal", "illegal car owner", etc... If a person illegally breaches a contract you don't call them a "illegal contractor". The term "illegal" is applied to undocumented immigrants because it fits a narrative.
>By breaking the law, these travellers got sorted into the "criminals" bin and are now treated like shit.
Going back to your illegally parked car, do we put the owner of that illegally parked car in the criminal bin and treat them like shit. Of course not, that would be ridiculous. Undocumented immigrants should be extended the same courtesy, and even if detained and put into civil removal proceedings they should not be treated as criminals or like shit for that matter.
> Not all undocumented immigrants are here illegally, but those who would be legally deported if discovered are.
Sorry, I don't know what your trying to say here. Any undocumented immigrant can be detained and subjected to removal/deportation proceedings at anytime...they don't need to commit any crime for this to happen.