Because if the government even thinks you might be a terrorist, they are not going to give you a visa/allow you to enter. And those checks are far more thorough

Because if the government even thinks you might be a terrorist, they are not going to give you a visa/allow you to enter. And those checks are far more thorough than the checks to board a plane. The only exceptions I could see would be some of those people might be allowed in as representatives of a foreign government, and there might be a few people who already have green cards and the US doesn't have enough evidence to remove them.