as part of Sabath Day observance (
https://www.lds.org/ensign/1978/01/mormon-sundays?lang=eng). I don't know if all, but many pioneers didn't travel on Sunday while trekking across the country. We also have no problem with full time church representatives dining out on Sunday's when their travel schedules necessitate and we seem to be ok with it since they're doing the Lords work. We also have people who have to work every conference Sunday.
If BYU sports are the 2nd or 3rd largest missionary tool, maybe they should be afforded some of the same allowances we give to others.