Because coronavirus doesn't magically disappear when you go outside? Yes, transmission is less likely outdoors, but if your face is a foot away from someone else's that pretty much negates the benefit of being outdoors - not sure what is so hard to understand. It's why the health advice has always been that you must social distance even if you're outdoors and even if you're wearing a mask, because the best way to avoid transmission is to just stay away from other people, pretty simple.
Also, there would be absolutely nothing rude with saying that you can't take pictures because it's not safe, especially if that person isn't wearing a mask. Zero people would've criticised her for that - we all know she would've got praised for it lol.