While the percentage of time is surely low, the high concentration of people per area means the frequency with which you encounter it can be quite high. I personally don't care much about being in random people's selfies, but having to constantly dodge people taking random pictures is definitely an annoyance when you're already trying to navigate a crowded space.

The easiest thing to do would be not care if you are in a random person's selfies at some big public event. They surely don't care.