Legal duty? Not at all. Moral duty? Thats a matter of opinion.
Then there is the strategic question where we give money to a "charitable cause" in order to garner some benefit from another country which gives us a strategic advantage and that opens up a lot of doors in terms of what charitable causes we could undertake.
For charitable work within the country I would argue it is constitutional for the feds to tax and spend that tax to help a certain city or state after a natural disaster as that could easily be described as falling under the general welfare clause. For money to be spent on foreign charitable causes you can at least argue that the constitution doesn't say the government is allowed to do that.
Ultimately it is for the voters to decide by who they elect as those representatives then decide how to spend money. We could certainly go the route of zero foreign aid. Famine happening in Africa? Who cares. Genocide happening in Asia? Not our problem. I don't think most Americans are fine sitting back and letting that happen without any intervention.
The government is not intended to be a charity. It never was and never will be. Treating the treasury like a never ending stream of cash for anything and everything is what has brought us to this point.
The government should not be a charity. I do not want my money wasted on a specific program that my representative votes for along Democrat party lines (the norm). I don't care how many members of Congress vote to spend that money.
When you give your own money to help others, that's called charity.
When you give other people's money to help others, that's called theft.
Elon posted about the choices we have as US citizens.
Would you rather have the government send a check/bank deposit to every voter
in the amount of $5,000, or
would you rather continue funding gender studies in Pakistan?