It's really a myth that America subsidizes others military budgets, and it's one that's actually caused political damage by now. Nobody needs the Americans to police the world, and many don't appreciate it. The US military acts in the interest of the US, and so is not policing to the betterment of anyone else. And if you want to talk about backing up budgets, it's entirely unnecessary. The EU nations out-spend their primary rival (Russia) and are more than capable of repelling them. Japan, Korea, and SEA nations together can defeat or at least hold off China. South America is somewhat stable in terms of potential for war.

In the end the US spends so much so it can retain global dominance. They've even said it. It isn't needed to protect other countries, they don't even act as police (they don't do peacekeeping anymore), they only act to impose their will.