The thing I've been wondering (and I could be wrong, it's just my understanding) but is it not illegal for an international aircraft to be landing at any field in the US that does not have a US customs officer on site?

I was asking a friend of mine who is a full scale instructor about flying private planes into the US and he said that upon entry you must land at an airport with a US customs office, you are not even permitted to exit your aircraft until the US customs officer instructs you to do so. Once you are in the US does that still apply? Surely it must or how would anyone know they've gained legal entry into the United States?