Because we already volunteered to do this specific thing, so it's incredibly disruptive for us to back out at (or well after) the last minute. Other countries do other things, and it would be similarly disruptive if they pulled or froze funding and operational support with 0 notice.

It'd be like if Microsoft donated free copies of Windows to a hospital and then one day was like "Actually we've been donating too much stuff, your licenses are invalid as of today, your computers will now not work." It would be similarly strange to ask why Microsoft's contribution is so critical to anything outside their own company, when it's because they made it that way on purpose, partially for PR purposes, partially from genuine altruism, and partially to tie Microsoft's well-being to the hospital's.