Bill Gates' Gimp
The only problem with masks being optional is that wearing a mask protects others more than yourself. So if you choose to wear one, you are protecting everyone around you if you happen to be sick. The one guy in the store that decides not to wear one is choosing to not protect those that are trying. So, as I see it the problem is that in places where it's optional it just gives people the option to put others at risk, not the option whether or not to protect themselves.