Many of those countries use AKs, but if you want to buy from the U.S., that means you buy American (with some very few exceptions). If you use U.S. assistance money, you’re definitely buying American. The U.S. is a very desirable source of arms because of all the things mentioned above - the support package that comes with the sale. Many countries also like M4s for their Special Operations folks, for the same reason that we use them - low recoil, lightweight, and very effective in close quarters.
I wouldn’t necessarily say that FMS usually involves countries buying from us with U.S. money, but you’ll definitely see that in countries where we are supporting them financially, like Afghanistan. In contrast, there are a lot of big sales, like 1,000 MRAPs for Bahrain, Abrams for Saudi Arabia, or C-17s for UAE, where those countries are putting up big cash money themselves. Then there are countries like Pakistan, where, as I mentioned above, certain sales programs are funded with our money, others with their own money, and some with a combination (they kick in some to make up for a shortage in U.S. assistance funds).