If “protestors” are on a Federal Interstate it is a federal crime. State routes and highways are state concerns. Most Interstates that I know have it prohibited that pedestrians are allowed to be on them without being in a moving vehicle. There should federal officers arresting all pedestrians that are supposedly protesting on Interstate highways. This circumnavigates local DAs and governments to take care of the miscreants.
Forget the above post. Interstate highways are federally funded but owned and operated by the state.