It doesn't matter if he dealt with those injuries before. The point is that he treats a helmeted rider, he does not know what his injuries would have been had he not had a helmet. And when he treats a helmetless rider, he has no idea what his helmet would look like had he been wearing one.

If he treats a helmeted rider and next a helmetless rider, and the helmetless rider has worse injuries, he can't say whether that's due to not wearing helmet or just because the helmetless chap got into a more severe collision. Even knowing circumstances of the crashes does not always help, because two very similar-sounding crashes can produce drastically different injuries. Moreover, even consistently seeing helmeted riders with less significant injuries on average is no proof for anything. The answer could be as simple as the fact that riders who wear helmets are more likely to have average or above average income (and thus have health insurance) and concerned with their health in the first place, while helmetless riders are more likely to be poor (no insurance) and not as safety-conscious. As a result, the former will see the doctor even when their injuries are relatively minor, while the latter are more likely to avoid a visit to the doctor unless things are really bad.

That's all fine, of course. But you're using a completely unsubstantiated claim to support your view, that's all I'm pointing out.