As for foreign students, there are fantastic foreign medical schools around the world... The fact that a program may accept more may be due to the fact that they have had residents from that school before and liked the results. Or they like the program to be multicultural. OR how about the fact that the applicant was competitive and they liked them? LOL.
Let's be honest here. Top programs in the US will take an FMG who is strong and whose training is considered on par with American training. At each Ivy league institution you will see a few FMGs, and all will be excellent. But the fact is that out of say 50 residents in a program, 5 will be FMG.
However, if you go to other programs at the lower end of the spectrum, you'll see mostly FMGs, Caribs, and maybe the occasional DO (though the latter probably not very frequently since DOs have their own match). It is because 1. American grads have better credentials and training and thus better options 2. The programs don't really care about resident quality and want to take whatever comes for financial/rostering reasons.
In my experience I've seen that programs that rely on heavy FMG recruitment tend to have really poorly-performing residents and very weak educational systems, with very poor boards pass rates. I've met residents from other programs at my training institution who could not speak English and did not know the basic tenets of medical practice - if I were a staffman at that point I would have raised a huge fuss about it because it was certainly hazardous to patient care, though each of them did fail the rotations. I would not be surprised if these FMGs bought counterfeit degrees abroad and jumped through the few hurdles required to get a shot at the system.
Call me a hardliner, but I'm all for blocking the match off to anyone who hasn't obtained an LCME accredited medical education. None of this FAIMER garbage - too much variability in quality. If a foreign national wants to practice medicine in the US, they are very much welcome to write the MCAT and apply to medical schools here.
The poster below makes a good point about checking out board scores rather than generalizing. Patterns are patterns, and though nothing is absolute, one must have their radar a bit more finely tuned when assessing FMG-heavy programs.