They're willing to do that because the US is good for the money. It's considered a secure investment.
The trouble is not paying off the debt - they've got that covered, otherwise nobody would be willing to invest. The trouble is actually the deficit that is incurred by making good on these bonds. That's the bit that actually hurts us.
Edit: also ironically, screaming about a collapse makes the collapse more likely to happen. If people abandon the dollar because they think it will collapse, that will actually help cause the collapse.
Self fulfilling prophecy...