For the vast majority of applicants, Section 214(b) of the Immigration and Nationality Act applies. Foreigners seeking to travel to the United States, whether for medical treatment, study, simple tourism or anything else, must obtain a visa. For most applicants, this involves an interview at a U.S. embassy or consulate. For all of them, the challenge is to "overcome immigrant intent" during their visa interview. In other words, they must prove to the consular officer reviewing their case that they have a legitimate purpose of travel, will respect the laws of the United States and the terms of their visa, and return to their country of origin. If you read that to mean "guilty until proven innocent," then you are correct.