Late af, but the whole point is, you can't really pinpoint when that is and isn't the case. It's just your shitty judgment that tells you it must always be the case.
I'm not denying SOME people don't have an "agenda", but why is that even bad when they do?
If someone doesn't have a preference for skin tone themsleves (e.g. they don't find darker or lighter skin more/less beautiful), and thinks it'll help people feel more beautiful to draw their characters in more diverse skin tones, why does that hurt anything? What's the big bad problem with that?
Personally though, I think the number of cases where people have preferences is just higher than you think. ¯\_(?)_/¯