Like many markets these days, US debt is a greater fool market. In the case of our debt the greatest fool of all is the Fed, or more precisely, the American people who tolerate the Fed printing money to buy debt and the politicians who benefit from it. The simple reason that any foreigners would buy our debt right now, or at any moment in tine, is that they expect to resell it. Some greater fool will buy it from them.
The only other reason for buying US debt is the expectation that US income tax slaves will be able to pay it back. That is potentially a sign of economic recovery but that pales in comparison to the foolish recipe for economic failure promulgated by the Fed: funnel printed money into Keynesian spending and large banks to trickle down into the greater economy. They ignore the fact that undermining the currency penalizes long term investment and encourages speculation and bubbles.
I looked into it somewhat in hope of finding out if the cost of the treasuries we're selling China and other countries is less than the face value. That is the nature of these types of investments, but to be honest, I couldn't confirm one way or another. If you have something showing the foreign countries are taking a loss on our debt, I'll thank you in advance for posting it.