> It is necessary to a functioning society, it reduces corruption, and it helps the outsider. Also, the community benefits from having new blood - it will wither and corrupt itself without it.
Citation badly needed
> It's written declaratively, but that is no indication that it's true or right.
You don't say...
It's written declaratively, but that is no indication that it's true or right. IMHO communities do have a strong moral obligation to be open to outsiders: It is necessary to a functioning society, it reduces corruption, and it helps the outsider. Also, the community benefits from having new blood - it will wither and corrupt itself without it.
> that is morally equivalent to an invasion or colonization, for lack of a better term
Colonialism is not about outsiders coming inside, it's about power. If in the 19th century a bunch of people from southern Africa came to the UK, the UK wouldn't have been colonized (obviously). The southern Africans didn't have that power, and thus couldn't have used it to inflict the associated harms on the people in the UK. But the UK did have the power and did inflict the harms.