Its like recruiting in the military.
Many of the tests that we took at the career counselors office, as well as the usual IQ tests, and the DSM that we use in Psych, were invented by the military. They were meant to select and place recruits in the jobs that they were best fit for, of course with their own personal preferences as well. So the recruit sits in the recruiter's office and tells them what they want to do -- "I wan't to be a flight surgeon". The recruiter says, -- "well your IQ is XYZ, and your undergraduate GPA is 3.2. We can't get you into the Armed Forces medical school, but we can train you to be a combat medic." You see, an unrelated test, and outside college experience is being used because its part of the recruit's personal history.
Even if the USMLE wasnt meant for residency selection, it is part of our personal history. If I was convicted of a felony, that conviction wasnt designed to be used against me in getting a residency. But it certainly can be, same goes for work experience, or a PhD. Whether a test, or an essay, or a research experience was MEANT to be used for resident selection is irrelevent. The fact is that it is part of the residents personal history.
Just like a military recruit, a residency applicant comes with a file containing their personal history - USMLE scores, outside research experience, past job experiences, other graduate degrees of any kind... PDs get to look at your personal history and determine if they think that you would be suited to be a resident in their program.
It is their job to decide what personal history traits will be required for entry in their program. It can be USMLE scores above a certain level, or only American grads, or only people who have graduated med school less than 5 years prior. Whatever it is, some of your personal history traits will help you, and some will hurt you. Thats LIFE.