The author is stating they don't do it because they believe the US is a good place (or not) to place their money (the usual criteria you would use when deciding amongst competing choices of where to put your money) They do it as a mechanism to achieve another goal: an artificially low value of their own currency. This is so their goods look cheap(er) to other countries. That in turn keeps employment high and social unrest low. That means less resistance to one party rule.