Ultimately, this comes down to two things:
1. The guy arrested was probably just trying to be a good person and was in the wrong place at the wrong time. Unfortunate, but life.
2. Police have the right to arrest anyone that they have reason to belive is breaking a law that they are meant to enforce. When you get let off by an officer, they're using their best judgement to realize that that law you just broke is irrelevant in the current situation, or is bad for the common good.
An example would be the recent incident in Miami where a passenger was shot by an Air Marshall. He was probably an idiot who got too riled up, but when he ran, the officer was completely in the clear to shoot him. In the public's moral eye, he got a bad break, which we automatically attribute to unfairness. It wasn't unfair, just unlucky. (I'm not saying the subway guy was a terrorist, just drawing an example). Thats why bums can sell tickets all day long, but a helpful guy who does it unknowingly once gets nailed.
And then there's corruption of law, a far too delicate subject for uninformed people to yell about over the internet. We don't want "The Flame Heard 'Round the World"...
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