Looks like the teaching of philosophy really has deteriorated, this in an age where people spend over 20 years pretending to be having a formal education!
First the "deal with it" argument and now this unwillingness to understand basic facts about morality and community.
Who decides? It has to be first an individual thought process, then that can be examined and refined by public discussion (as we are doing).
What is moral/immoral? Well, that is a big question. How do you think we should decide?
Morality does not arise individually. You learn it, take it, from your community. You can change it, pick and choose, even rebel against it,. but its a community product. And this is the beginning also of why community is important.
Morality is socially determined. Behavior is individually determined (yeah I know free will and all that but let's not get into it, please...)
So you could start with some definition of utilitarianism. From there you could ask whether freedom of movement would increase the sum of happiness and well-being / reduce the sum of unhappiness and suffering. It seems to me obvious that it would, not least as it would reduce the incidence of wars if everyone had friends and family in all corners of the world. How to get there is certainly a question for a utilitarian, as it could cause disruption that harms people's well being, but the goal should be clear from this foundation.
And speaking of philosophy bringing utilitarianism into arguments is regarded as the last refuge of the scoundrel, as it can be twisted into justifying everything.
Others may take the teaching of Jesus as their axioms. It seems the most relevant would be the parable of the good samaritan. From that we could derive that we should treat those who come from geographically remote locations, are genetically distinct and have different cultures as neighbours.
Both of these sets of axioms come to the same conclusion, we should not discriminate based on geographic origin, genetics or culture. As I said in the 2nd post of this thread, I am not aware of a set of acceptable (to me) axioms that would result in a different conclusion. I would be very interested in how you would derive a different conclusion. It is worth noting that I do not accept "everyone else thinks X" as any proof of X, as it has been shown to be wrong too many times, both historically and personally.
One of those moral systems has been around for hundreds of years, the other for thousands, and communities along with its necessary border conditions, those who are in and those who are out, have existed for all that time. Clearly these moral systems coexist nicely with community membership, which be definition means both inclusion
and exclusion. You cannot just invoke christianity or utilitarianism and then state that they are agains borders of any kind, because they have been developed, spread, and exercised in societies with lots of borders.
Utilitarianism is hopeless as a moral system, I'm not even attempting to argue about it. Let's just say that the nazis were convinced that "clearing land" for the "superior people" was an utilitarian endeavor and leave it at that.
Christianity is an interesting case. It's universalist in outlook, but you also have the give unto Caesar what is Caesar's - it was already tainted with having to deal with politics even before its supposed founded got nailed (pun intended) over being politically inconvenient. Christianity as it developed never had any problem with gated communities: guilds, cities, kingdoms, national states. Welcoming a neighbor does not mean granting community membership to a neighbor. The neighbor welcomed into christianity would be unable to continue worshiping Zeus, or Isis. He would be required to
convert to a different culture in the process. And if you want to invoke the Jesus of the gospels and say that christianity strayed from it, remember the cursing and the demands, not just the brotherhood under god. Remember the conditions for access to the kindgom of god...
Communities
have to have borders, they can't just be open to anyone and everyone without requirements else they would cease to be a community. To expand of this discussion: this was and remains the silliness of the people taken in with the talk of "multiculturalism". Even in the cosmopolitan places that are usually given as examples of "multiculturalism" (think Venice of old or any other big trading place) there is a a set of laws, a power, or sometimes several sets and very well-demarcated communities each with its own. And those are the worse places because you can't jump from any group to another without risking being denounced as traitor, ostracized or even killed.
When embracing the current common idea of open and happy multiculturalism you idealize a false myth that never existed in history. Multiculturalism where it existed in some stable way was either closed, policed (think ghettos and quarters, or the ottoman system of separate laws, that kind of stuff...), or not peaceful at all. And sure you can end up with a synthesis but that means you drop the multiple cultures and created a single new one. And the required set of changes is usually traumatic to those involved.