Since when do the police need to suspect you of a crime to be able to "interview" you? Extending that logic fully would prevent them from being able to talk to anyone, at any time, about anything, unless there is a suspicion of criminal involvement. There is no substantive difference between just talking to someone and "interviewing" them.
Point 2: Police can interview people if they believe that person has mere KNOWLEDGE of crimes, not active involvement.
And finally, suspicion of offenses relating to "immigration or citizenship" ARE crimes, and are justification enough ON THEIR OWN for "interviews" to take place.
From the article:
Acting Police Chief Andrew Kirkland said Tuesday the U.S. Attorney’s Office in Portland asked for city police cooperation last week. He said police denied the request, citing an Oregon law that says no one can be questioned by police unless they are suspected of being involved in a crime.