I don’t think that outdoor activities are risky enough (regardless of their purpose) to present a significant threat to grandma, so I say play ball.
i think a lot of the protest footage i've seen is relatively low risk, but some of it is much higher (tightly crowded, yelling), even if outdoors.
also there are associated behaviors that are likely higher risk than the actual protests. like there was a big story from dc (i think) about some dude that let 50 protestors stay in his house.
a little different, but associated law enforcement actions also promote transmission. have heard of large numbers of detained protestors crowded in jails, etc.
btw, i'm not saying i think we're going to see a big explosion of transmission from protests. i don't. but it pretty clearly elevates the risk of contagion for (some) participants and those they subsequently contact.