We just put them on the market and buyer buy them. We do not "keep them buying" securities - there are not government sales staffs that go out and peddle securities and they are traded after sale openly on every continent. If there is a benefit there will be buyers. It is as plain as that. Politcal manipulation of securities does not work.
On top of that, almost all other world economies are significantly export led. It makes no sense to destroy your customer even if you could.