isn’t ‘Pointing out ‘suspiciuos’ people’ on a public forum rather immature by itself?

Not in my opinion. They were just people trying to catch the bombers. No one was jumping to conclusions from what I saw (maybe some did but they were downvoted further down than I scrolled), rather they were just pointing out scenes that looked suspicious to them so that the police could investigate further. While there have definitely been cases in the past where a line has been crossed, I don't believe that to have happened this time.

For the most part, they were pointing out suspicious pictures, in the wake of a nearby large-scale criminal event. Not suspicious people by name.

What were the pictures of?

Would the pictures, or the people in the pictures, be suspected of being bombers? Were the police looking for pictures or for people in pictures?