Professors do hand out F’s to any student who fails a course, especially in STEM courses. If you’re failing, you’re failing, but most students drop or withdraw if they’re not doing well.
In courses that involve mostly writing, the grade might be more subjective. Some professors are tough on papers, and others are easy. Since lawyers must craft an argument in writing, and they don’t need STEM degrees for law school, some might get into law school with an inflated GPA. But, that would be true for any student of any background.
What would happen with a professor who flunked a much higher proportion of his black students than white?