> I don't know if we can speak about their motives, but they certainly contribute to overall stability.
"For the world" is speaking to motives. America isn't bankrolling "stability for the world", its bankrolling stability where it suits the interest of American decision-makers, and instability where stability doesn't those interests.
> The US is very interested in global stability
Except for those places where its interested in altering the status quo, in which cases it spends lavishly on doing that.
> you can argue that's also in their own interests as well
"Their own interests" -- that is, those of American decision-makers -- are the only factor, not an incidental "as well" factor, in the decision of where to bankroll stability and where to bankroll instability.