You are concluding things as fact that are not actually known to be fact. The behavior you describe, for both participants, cripples their score. In reality we don't know why the guy running chose to run. My suspicion is that he simply lacked the confidence to engage and didn't want to get shot down, but I can no more claim to know that than you can claim to know that he was running due to score.
I won't say that score is the only motivation for running but it can be a big part of it. Most people don't understand how it works anyway so to them not dying at all costs will improve their score.
Replying to an earlier comment about declining numbers: Perhaps this should be a taken as a sign that changes need to be made? I am excited beyond your wildest imagination about the new terrain engine. Every time I see a new video on the main page I squeal like a schoolgirl. But as wonderful as it is I think that unless we change the way the game works it won't bring people back for more then a tour or so to check out the new features before they get tired of the same "Chases High" experience that we've come to expect.