Should Professors and by extension their TA's, hold student athletes to the same standard?
The following is specific to my university, the University of Michigan, but I would guess that the situation is similar at many other universities.

As far as I know, I am not allowed to apply different grading standards to different students in the same class, except that, if a class has both graduate and undergraduate students, then the graduate students may be held to a higher standard. (I've never used different standards for grad and undergrad students, but I believe it is permitted.) 

Despite the general rule, there are oft-repeated stories about a particular professor (not in my department but in my college) who allegedly applied easier grading standards to athletes, over a decades-long career, and apparently got away with it. I don't know whether a tenured faculty member could get away with it nowadays, but I certainly wouldn't advise a TA to try it.

Apart from grading, though, athletes in major sports have one advantage, which may compensate for the time demands of their sports. The athletic department provides tutoring for student-athletes and tries to keep an eye on their academic progress (partly so that they don't become academically ineligible to play, partly to keep up a respectable graduation rate, and partly for the genuine benefit of the students).  I once had two varsity hockey players in my class, and an assistant coach phoned me (this was long before email) to check on their progress and to ask me to let him know if they had difficulties in my class.