You are making the assumption that attending such a course makes you a lesser risk than those that haven't. Conversely, though, it may be that it indicates a proclivity for speeding. Not having attended one is pretty likely to include both those not inclined to speed in the first place, and those that just haven't yet been caught.
What we don't know is what insurance company actuaries are telling them about incident rates, or claims numbers/values, between those that have attended such courses and those that haven't. And that, of course, determines risk.
But we, and the insurance companies, both know that IF you attended such a course, it was because you committed the same offence that, but for such a course, would have got you points and/or fine, so they're entitled to treat that information the way they do, and react accordingly. It's the nature of insurance that those offering it determine risk, and therefore premium required.