How did this travesty pass 4th Amendment muster? "Yes Mr. Bar Owner, a few of your patrons stole a car a few weeks back. We'll need the names and addresses of all the people who visited this place for the last year please."
I'm not sure the fact pattern you're providing really illustrates the concern.
Rather, I think the concern is that the prosecutors have what appears to be a very diffuse criminal inquiry, and that the specific information they're looking for has a clear side-effect of intimidating lawful protesters. The concern is that the "investigation" is a pretense for intimidation.
You can't say that about the police investigating bar patrons for a specific car theft or whatever.