I used to be like you man. Casting my judgmental glare at other gym-users and hobby joggers I felt weren't doing it properly. Their form, or level of intensity, did not match mine so I snickered at their pitiful excuse for exercise. Then I hopped on Letsrun and joked about their ineptitude so I could feel superior. But here are two reasons you should cut that out:
1). You don't know that guy/girl on the treadmill walking 2mph. You don't know what else they've done that day. You don't know what physical limitation they might be dealing with (like a heart condition). You don't know how long they've been working on their fitness or what their aspirations are. If you actually know someone, you understand their situation, and you actually have the kind of relationship with them that warrants the dispensation of fitness advice, then maybe you can speak up and set them straight when you see they are doing something wrong or pointless. But even then, check your motives and think twice. If you just genuinely want to help someone get better and they are open to your advice or encouragement, great. Otherwise, keep it to yourself and get on with your life.
2). If you feel superior to a person just because you exercise better than they do, and you jump on a message board to poke fun at them with your anonymous running buddies, you're kind of a loser yourself. One guy is walking 2mph on a treadmill. The other is watching him diligently and posting about it on Letsrun. Which one is the bigger loser here? Seriously. No judgement from me though, man. I've been there. But I see that in myself now and I've tried to stop being such a petty jerk. If I run a 5k in 18min and comfortably beat the majority of other participants who walk half of it, WHO FUGGIN CARES?!!
As others have stated, at least the guy is doing something. He's making an effort. We don't know his situation.