And I do admit that all the writing on this that I've seen was biased in nature, but it still seems like these three are being questioned for everything they know about other anarchists and activism in general in the area... not just about this specific crime that is supposedly being investigated.
As sluissa said, this is a grand jury investigation that appears to have a broad scope.
The purpose of a grand jury is to investigate *something* to determine what individuals can be charged with what crimes. Sometimes these investigations are nebulous, focused on general groups or organisations rather than any particular events or individuals.
In this case it seems it's a grand jury investigating general violent crime related to anarchist groups in the Pacific Northwest.
I would note that these specific people had specific crimes (even if not their own) they were being questioned about. That is, the property damage at the protest and the federal crime of crossing state lines in order to riot. Something in the investigation suggested these people might have information related to those crimes, so they were subpoenaed to testify.
It's possible a that intimidation of such groups was a (misguided) element of this decision, and that the investigation has illiberal intentions. But given the public facts about this it looks pretty much the same as due process from here.