Since Trump hates people who can't pay their bills perhaps he will encourage Russia to invade the poor of south Carolina you refer to lolThere's been a longstanding perception among certain factions of voters that the U.S. has been shouldering the lion's share of the burden of defense for the entire free world. Some Americans see it as a source of pride and national honor - the idea that America "saved the entire world" in WW2, an idea which was actively propagated among Americans all during the Cold War in order to engender and encourage public support for US militarism.
The same perception also heavily implies that America is doing it solely out of the goodness of her heart, that we take on extra expense and extra risk to ourselves - all because we selflessly care about the rest of the world and have no thought for ourselves.
At least, that's what the US government has wanted Americans to think about its military adventurism. Many Americans are still skeptical about it, such as the skepticism expressed about America's war aims in Vietnam or Iraq or Afghanistan. They don't think that the government's stated pretexts were strictly on the up-and-up.
But on the other hand, those who have actually grown to believe the government's BS over the past decades might perceive it as America's government being conned and suckered into sacrificing America's interests for the sake of other countries' interests. It leads some Americans to think "Why are we bending over backwards to prop up the world system when America is suffering and doesn't benefit one iota? What's in it for us?"
For its part, the government just pays vague lip service to arguments related to "national interests," but in fact, the government has never given an honest answer to questions like that since the Vietnam War (and even then, it had to be a "leaked" answer).
Also, more than a few Europeans have expressed a degree of resentment about American hegemony and the fact that we have so many bases and troops in Europe already.
So, the consequence of government propaganda leads Americans to think that this is all some selfless sacrifice that the Europeans neither want nor appreciate - and all the while Americans are suffering in economic hard times. In a state like South Carolina which is at the lower end of the economic scale, where many people are living hand-to-mouth and barely scraping by, Trump's statement of "let them pay for their own defense" falls upon welcoming ears.