It says that to avoid lawsuits. Virons on a used mask prove that there is some protection. Drawings somewhere on the Internet show the amount of virus that is transmitted with and without mask when a person is talking. That’s if the person has the virus, an active case. And the drawings also show the effect on the person being talked to with and without mask.
The amount of time the talking takes place is also a factor in the danger.
Talking loudly or shouting, singing, loud laughter, etc., are also factors that increase the danger.
I haven’t yet seen anyone wear a mask that prevents air from going in or out through the gaps on the sides. Plus, people have no eye protection. A mask is simply useless.