I think there are at least three overlapping bases for these obligations. The first is that the US is sometimes responsible for the fact that someone has become a refugee. For example, people in Iraq and Afghanistan who have helped American forces by serving as translators or in other capacities have sometimes been put at risk because of this service. There are already stories of such people being excluded from admission (and hence, safety) as a result of Trump’s policy.
The second basis for the obligation to refugees is simply the humanitarian duty to help people in desperate straits when one can do so. This duty has its roots in many different religious and secular ethical traditions. The United States has traditionally admitted more refugees than any other country (although Germany has clearly passed the US in this respect in the past few years). The complete ban on refugee admissions for four months and the subsequent reduction (by half) of the number who will be accepted is a failure to meet America’s humanitarian obligations.
The third basis for the obligation to refugees is that the United States and most other countries have acknowledged that the international state system has a duty to protect refugees. In the wake of the failure of democratic states to protect Jewish refugees from the Nazis, the United States led the effort to create institutions that would prevent such a moral failure in the future. That regime already suffers from severe limitations, and the new Trump policy will undermine it further. [...]
First, in imposing restrictions on entry that take immediate effect, it violates norms of fairness. As everyone knows, the policy has stranded people abroad who had already been living in the United States as well as people who had been given permission to come and had made life plans on that basis. To deprive people of a right to enter the United States that they had previously been granted and which they have done nothing to forfeit is unfair. [...]
Second, the policy violates the moral principle that it is wrong to discriminate on the basis of religion. The seven states whose citizens are not permitted entry are all overwhelmingly Muslim. Trump himself has implicitly acknowledged that it is wrong to discriminate on the basis of religion by denying that he is doing so. This recalls the old saying that hypocrisy is the tribute that vice pays to virtue. In this case, however, the hypocrisy is so blatant, given Trump’s past and present statements about Islam, that no one who cares about reality can take his protestations seriously. This policy is clearly and deliberately aimed at restricting the entry of Muslims.