This is the intuitive case. It is not all of, but much of why we open borders people are where we are. What is our reaction to people trying to make an honest living here, because their life back home sucks. Haiti and Cuba are the clear examples.
Mr. A, from Country Z makes $0.16 per hour ($2/12 hour day) living in his home country, and because of the kleptocratic government and extra-crappy institutions, he sees no future wherein he makes more than that. So, he takes both items that he owns, sells them to his neighbor, and leaves, heading for country Y, in which unskilled laborers tend to make $6.25 per hour ($50/8 hour day), by simply being willing to do work, with the intention of doing work.
Case 1: Mr. A arrives in country Y, finds regular work, rents a room from someone, and life is good.
This is a pure win for pretty much everyone.
Case 2: Mr. A arrives in country Y, and finds very little work, averaging only $10/day of work, but that's enough to pay for food, and still save half his salary...even if he can't find a room. Maybe he finds a room in a poor part of town for $50/month.
This is still a pure win for pretty much everyone.
Case 3: Mr. A arrives at country Y, and is sent home
This screws A, and the people who hire A to do their work, the landlord, and the taxpayers.
Case 4: Mr. A arrvies at country Y, and is thrown in Jail
This is a win for Mr. A (It's better than his old situation), but a loss for country Y. In this case, Z and Y are indonesia and Australia respectively.
Case 5: Mr A arrives at country Y and is shot.
Case 5.5: Country Y makes it hard enough to get in that Mr. A dies trying to find a way in.
Nearly equivalent to 5.
What will/do people do? In all cases, people basically take some (erroneous, self-favoring) model of the probabilities, sum the product of the probabilities and the payoffs across likely results, and life is good.
From the calculus of Mr. A...the only thing that would prevent him from doing the trip is: high probability of the case 5s.
Case 3 sucks, but no more than before. Case 4 is a win for A.
10% case1 + 10% case2 + 80% case3 is still a win.
From the calculus of a anti-immigrationist, the only thing that's going to stop a relatively steady flood of immigrants is a high probability of case 5s...which is monstrous.
They're coming for opportunity...the laws aren't going to stop them from coming. Now what?