The idea of illegal immigration was interesting because it's a derivative of the root and somewhat abstract idea of property. It can be something tangible or it can be ideological or even emotional. When this property is taken or violated, we have a negative response. A good example would be the current Star Wars controversy. The original Star Wars is a trilogy that a lot of people enjoyed. It seems to be about as momentous for them as the coming of anime fight scenes was for me. So when this ideological property is violated there is an immediate reaction to try and control the situation. Set the stakes. Preserve the borders. Ensure integrity.
Many people feel this way about many things. We even consider our children, in some ways, property. It's only the child's (well, for most kids) increasingly hostile reactions to attempts at control (at least parental control) that begins the process of the adult being weened from their sense of ownership of the child. (it sounds like a second birth, doesn't it- the child's affirmation of autonomy? It kicks it's way out of the inner-womb, and then kicks it's way out of the confines of parentage, or the outer-womb. It takes two births to be a man...or a woman)
This isn't about the national policy on immigration, but about the underlying concept of property. It's hard to pin down what's considered the very first thing a person identifies as their property. I imagine it would have to do something with their personality. Some people might consider their own awareness or ego their property sooner than another would consider the maternal figure in their lives. Some might even become possessive of their body faster than their sense of awareness or the maternal figure. I imagine it comes down to what each person prioritizes. Therefor, in disputes of property where you don't have an overarching legal code, it might do each side better to understand everyone else's priorities.