Kid Charlemagne said:
A characters actions should define their alignment, their alignment should not define their actions.
I don't disagree with you at all and I doubt that myself or any of my players would think of it in the way I described above. We spent a dozen years playing Rolemaster (which doesn't have an alignment system) prior to switching over to 3E when it came out so we're used to thinking of character motivations, not alignment. I just thought that it my be an interesting direction to approach the issue from.
Looking at the situation again from a Neutral perspective, say I'm playing a hard-bitten, self-serving Neutral Rogue. I witness this scene start to unfold. I figure that I could get the guards involved, but the less I have to do with the law, the better. If I intervene and attack the guys delivering the beating, I might be making a powerful enemy so that's not a hugely attractive option either. So I walk away or hide nearby and watch what happens and listen to see if anybody drops a name or other useful info. Sounds fairly Neutral to me, if shaded a bit toward Evil.
OR, I decide that, by intervening and saving the person getting the beating, I will almost certainly gain an ally who will be greatly in my debt. Maybe he's got money or connections or influence that I can use to my own gain. So I attack the thugs, drag the unconscious guy into a nearby inn ("My buddy here had a bit too much to drink. I need a room to let him sleep it off.") and wait for him to come around. Then I tell him how I saved his life and barely escaped with my own but I'm glad to see that he survived. He says he's forever in my debt...yadda, yadda, yadda...Step 3: Profit!
So there I did the "Good" thing, but I did it entirely for personal gain. S'okay, I won't make a habit of it (the "Good" part, not the "personal gain" part). I'd still say that was a Neutral act, if shaded a bit toward Good.