It's pretty obvious you'd handcuff them as soon as you arrested them, because even if they are not resisting or trying to flee now, there is a real risk that they are only not doing so because they don't think the odds are good at that moment, but will do so if the opportunity arises. They might just be waiting for the officers to be distracted, or to be left along with only one officer.
None of that applies to the reporter, of course, but do we want police making that kind of judgement? At the very least it would raise thorny issues about bias in how they choose who to handcuff and who to not handcuff.