Based on  science and medical opinion the best way to "slow the spread" is through good hygiene, social distancing, and common sense (stay at home if you are sick).  The mask mandate not only limits private employers ability to determine how they want to operate their businesses, it limits an individuals freedom of choice.

There is currently no evidence that wearing a mask (whether medical or other types) by healthy persons in the wider community setting, including universal community masking, can prevent them from infection with respiratory viruses, including COVID-19 (Science Alert 4/9/2020).

At best, masks serve to prevent infected people from passing on infection, not to prevent the wearer getting infected (Science Alert 4/9/2020).