Foreigners come to the United States for all sorts of reasons. Some come for temporary visits to conduct business, visit tourist sites, or study, to name a few. Others are immigrating, and still others are refugees.
The bar for entry is highest for refugees. Typically, a refugee spends a couple of years after applying for refugee status while the claim is considered and checks are made. These include interviews, electronic checks, fingerprints, medical exams, and investigation of past history.
Obviously, our borders must be protected as far as possible from those who hate us and want to do us harm. We rightly guard traffic into our country.
To protect ourselves from any possibility of terrorism, however, we would need to forbid basically all entries—no business or tourist travelers and no foreign students learning from American universities, for example. Our country would suffer major commercial damage, not to mention losing influence.
Odd that refugees, who are vetted more than all others coming to this country, would be so feared.