AS such, the University attempts to aid in keeping the Sabbath day holy by not holding amateur sporting events, or basically any non-religious type event, on a Sunday.
As to pro sports, individuals gets to choose their career path, and some jobs require Sunday work. Sports is one of them, but so is the medical field and many others. But it is an individual choice as to career path, and not a BYU thing at all once you are in a career.
When it is how you provide for your family vs. just what you do for fun, it gets looked at differently.