Plenty of fair fights with no weapons end up with someone dead just the same. People should be able to defend themselves by any means, they just need to know when to stop. I'd prefer to not defend myself fairly, I'd want any unfair advantage to ensure I get to stay alive. If the pedestrian in the video got the upper hand, who is to say he won't bash this guys head into the pavement until the bicycle guy is dead?

Now looking at the video, its hard to tell exactly who is throwing the first punch, or if the pedestrian was moving in to attack, but at about 20 seconds into the video you can see the bicycle guy move away, and then move back in to land a few more hits while the pedestrian is staggering around. At that point the guy starts to lose any legal base for defending himself. The same thing happens to police all the time, they get into it, and don't know when to stop and then everyone says excessive force.