You are correct in that there are transfers of money and arms, the individual value and long term wisdom of which are always debatable. What I am talking about is a de facto subsidy, based on the fact that many other countries have in effect outsourced much of their security apparatus to the United States because they know that the US would step in for any existential crisis caused by mutual enemies.
Think of it this way. It costs billions to construct a single aircraft carrier. Many seafaring nations currently build smaller navies than they would if the United States were not filling a large portion of this role for them. When buying aircraft, ships, tanks, missiles, etc, an allied country only has to build those assets marginally necessary to create a comfortable excess over and above the security already seen as inevitably provided by the US. That constitutes an indirect subsidy.