This ignores the context of his neighborhood having break ins the week before, and seeing someone walk around the neighborhood looking at houses. Emphasis on the houses (as that is where the break ins took place, and what Zimmerman was focusing on with Martin. Notice the first thing he said to the 911 person is that he thought Martin was looking at houses). If you read the transcripts to the 911 call, he seems more concerned with his behavior rather than: I. His race. II. His age.
I'm not saying he didn't racially profile, but what evidence is there to suggest he was? And even if we use the logic you re-posted, why couldn't it be a situation where his suspicion had to with the behavior and not those other factors? Again, not saying it was that. Just that, if we agree there isn't enough evidence to suggest one way or the other, so we try to narrow down the possible factors that COULD have been and were the most likely the reasons why he was following him (and we try to pick the thing that is most logical), I don't see why "behavior" and "context of past incidents" couldn't also be a factor that played in his decision to follow him.