All human lives are equally important from a moral perspective .
But from the perspective of a country's government -- any country, not just the US -- their own citizens' lives are the most important.
Imagine a man kills a woman -- a random stranger he met on the street. It's a terrible crime.
Now imagine a man kills a woman -- his daughter. Most people's gut reaction is that this is a worse crime than the previous example, because a parent he has a moral duty to protect his child.
Likewise, a country is supposed to protect its citizens -- so killing its own requires a stronger justification than killing non-citizens.
I'm not going to take the discussion off-topic by taking a position on the circumstances under which it's justifiable for a country to kill its own citizens or non-citizens. I'm just saying that a country's justification for killing its own citizens needs to be at least as strong -- and usually stronger -- than its justification for killing non-citizens.
 This paragraph is completely off-topic. "All human lives are equally important" sounds like a good and simple axiom for a value system. But it's interesting to try to think of situations where any of several people could be sacrificed to prevent some great evil, and the group figures out reasons to sacrifice some person or people with certain characteristics in preference to others. For example, "save the women and children first" is often said in disaster movies -- which implies their lives are somehow more valuable. Based on this anecdotal evidence, it appears that real human value systems often don't follow this axiom.
Sounds like a useful simplification at most.
As I footnoted, that's really what it is. But I used it to start my comment, because the parent's viewpoint may implicitly rely on it.