I mean... in the South (Alabama e.g., not Mexico) many native born, very white Americans do not speak English in a way I'd call fluently, and the Brits question Americans' English fluency... my question is: if you don't know what a prepositional phrase is, a split infinitive is, or the difference between "its" and "it's," can you graduate medical school in America. And if you haven't graduated medical school in America, and you want to become BC in rad onc in America, you definitely need English fluency or 1) you won't get a residency spot, and/or 2) you'll never pass written boards.
So English fluency is actually a reasonable screening tool for admission to any US residency program, I would think?