The idea is that depriving, say, China of doctors doesn't really accomplish anything except generate more sick Chinese people, which doesn't really pose a major strategic advantage to the US. The only incentive to import them is if there is a major shortage locally (and so far there isn't).
On the other hand, depriving China of engineers can give a significant competitive boost to the US. The more researchers and developers you import the higher chance you have that something that would've been invented in a competing country is invented here instead.
If the US is short of doctors and dentists - and it is, otherwise their salaries wouldn't be so high - then there's no economic reason not to import them.