Unless you knew every place and at what time you were at, do you just assume anyone protesting at any point in time in the entire city is a contact?

The whole point is to find people you were likely around for some period of time (shorter inside, longer outside), right? To me, it just seems like a lot of noise.

Then, there's also the separation of concerns. If you want people to help you with your primary concern, you want them to not feel like they are jeopardizing themselves for other things; like the cop investigating a murder who tells a witness they aren't interested in their blunt.