Since the article posted seems to be an opinion piece, I looked up a news article about what originally happened. Apparently media need prior permission to take photos in Union Station in Toronto (I'm not sure if the photographer had permission or not, I would think the paper or the reporter would have gotten it taken care of ahead of time).
But apparently he saw a couple leaving the scene, the woman was crying and they told the reporter they didn't want to talk to him - so he ran ahead of them and took their picture, and the guy smacked him... that's how/why the police got involved. Not that the guy should have smacked him, and he may have been allowed to take photos there, but the article said the station was crowded that time of day, maybe it would have been a better decision to go take someone else's picture... Sounds like it was a mess of a situation.
I've had co-workers a couple of times that no sooner had left a home visit than the police went in for a drug bust in the same apt. building - they'd been watching the place the whole time. You never know what might be going on that you aren't even aware of. Good example about someone taking photos in front of a school, the person might be on public property on a sidewalk, but you don't know if it's someone's grandpa taking pictures the first day of school, or the neighborhood creep using the camera as a premise to hang out and try to abduct a child (or any of dozens of possible scenarios). You just don't always know the whole story or why you might be getting asked to leave an area.
Certainly there have been situations where a security guard or police officer may not have handled something properly, and I can see why that would make someone mad if they feel like they have a right to be someplace; with 'everybody' having a camera there seem to have been plenty of overreactions. It doesn't seem like arguing with an officer would end well or that the photographer would accomplish what they set out to do. I just think the end result would be better going thru proper channels.