Salman Rushdie isn't a HVT, he's just some guy that wrote book that rustled Khomeini.
Stop deluding yourself with American grandeur, they are as vulnerable as anyone else.
Millions of Mexicans cross the border to Mexico annually, they routinely bring in war criminals and former terrorists from the ME and North Korea without realising it. Their security apparatus is a joke, except around HVTs like Trump etc. There's simply too many bases to cover.
Sleeper cell operatives are often highly trained and blend in like everyone else.
https://edition.cnn.com/middleeast/live-news/us-iran-soleimani-tensions-live-intl/index.html#
The americans have literally admitted that Iran is operating in the US.