Could this be a factor in how other countries operate?
However, the fact that the US is the main component of NATO gives the US a lot of leverage over those other countries which is valuable in its own right.
It would seem natural to think your own soil is the only thing you can ever rightfully defend. USA seems to be doing the defending anywhere but on their own soil.
Countries like Poland and South Korea are within the reach of geopolitical rivals, though. So places like those are where the US is garrisoning much of its combat ready troops. Which makes perfect sense.
I'm also not so sure what you meant by "It would seem natural to think your own soil is the only thing you can ever rightfully defend." Why wouldn't it be rightful to defend the territorial integrity of other countries? In fact, the US is obligated to defend it as per international treaties like NATO.
Also, as a percentage of GDP US military spending isn't all that high. It's just over 3% [1]. South Korea is at 2.6%. In most years, Israel outranks the US (they probably could afford to drop their spending because their regional rivals are having very bad domestic problems and are in no condition to attack Israel). In theory, all NATO countries are supposed to spend 2%.