Charity ≠ one side of a quid-pro-quo was worthless.
The US gave/gives something that is fairly easy for it to provide, and valuable to the other side.
In return the US receives something that is of great value to it and fairly easy for the other side to provide (as long as the US keeps its side of the bargain).
If US security guarantees turn out to be worthless, you will see a lot more nuclear powers as well as a drastic increase in conventional capabilities.
The US will then be one power among many, rather than a mostly unchallenged superpower.
It's charity. The US spent the equivalent of many trillions of current dollars on nuclear weapons and delivery systems, and those under the US nuclear umbrella didn't have to. Withdrawal of that umbrella will mean these countries will then have to pony up money to replace it.
What did the US receive in return for this? Gratitude isn't worth much.
The US gave/gives something that is fairly easy for it to provide, and valuable to the other side.
In return the US receives something that is of great value to it and fairly easy for the other side to provide (as long as the US keeps its side of the bargain).
If US security guarantees turn out to be worthless, you will see a lot more nuclear powers as well as a drastic increase in conventional capabilities.
The US will then be one power among many, rather than a mostly unchallenged superpower.