That the immediate crisis is over and nobody burned is irrelevant from a moral action standpoint. That's like saying it's fine if the government evacuates only the people who make more than $100,000/year from a burning building, as long as the fire fighters manage to put the fire out. As long as nobody ends up burning, it's perfectly okay, right?
Wrong.
Here, let's try a more apt analogy:
After firefighters rescue everyone from a burning building, it's fine for the government to only allow those with passports to go to the airport and board planes going to another country, since those without passports won't be allowed off the planes in the foreign destination. The government will allow anyone to get a passport and then board a plane, but it will take weeks to get one if you haven't already done so. Meanwhile, nobody with or without a passport is left inside the burning building.
That raises a separate set of issues, which you don't seem to recognize. But how about: The government pulls everyone from a burning building, then shoves all the poor people without passports into a shelter across the street, where they don't have running water, and may even pick up some of those trendy medieval diseases that San Fransisco made popular again. Those with passports get to fly off to a resort.
Is that a problem?
Wanna compare that to the US border issue? Many more people die in Mexico and Central America than have died to this volcano, and those people are having a harder time getting in than just needing a vaccination. You think everybody should be allowed on the cruise ships, so should everybody be allowed across the border? We owe them a flight to a resort too, right?
Different circumstances. A volcano is a natural threat. In the parlance, an Act of God. Beyond human control, and thus those affected are widely considered to deserve our sympathy.
Mexico and Guatemala are politics and economics. Quagmires of misery, caused by humans but resistant to clear answers when it comes to causes, solutions, who's to blame, and pretty much anything else involved. Even worse, most interventions seem to make things worse, not better. (And yet, people remain extremely confident that
their solution will work (even if very similar solutions failed repeatedly in the past), and that anyone who refuses to throw all the resources they demand at the problem right away is Evil.)
That's why it's easy to get support for one, and not the other. They're treated very differently in the public mind. This isn't unique to these two circumstances, either. Look at the various causes of death. We take extraordinary action, trillions upon trillions of dollars and horrendous violations of civil rights, to fight a so-called war against terror. Which kills a handful of people compared to heart disease, cancer, car crashes, and all kinds of other things that don't get a fraction of those resources.
Or look at the various ways the value of a human life can be calculated. For instance, the EPA typically considers a human life to be worth about $9 million. That's the amount they'll spend on environmental measures that can be calculated to save a single life. Conversely, the median household income in the US is about $68K/year. Consider a working life of 45 years (start at 20, retire at 65), and that works out to lifetime earnings of only a bit over $3 million. And that's the median household income, not the median individual income, so the real number is considerably less. But even if we run with that number, we'll take measures to prevent the loss of life that at least triple the value of that human life to the system. And that calculation varies widely, depending on the type of threat.
This isn't an issue about how we value a human differently in different circumstances. It's about equal treatment, and human dignity. That, however we decide to value a life under different circumstances, when the government is purportedly acting for society, that they value all those lives equally. That we don't discriminate based on whether they're rich or poor, whether they're green or blue, whether they come from the right side or the left side of the tracks, or their personal beliefs. All people are equally deserving of rescue from burning buildings and pyroclastic flows.