Being able to forget skills helps us experiment with skills, so we can see how they work and what they're like without having a big commitment. I think it's decently well known that the majority of skills are almost useless for the first like, ten levels or so. Losing the ability to forget them well before players are able to get any use out of them doesn't really make much sense to me. There are a number of things IC that players have very little knowledge about how to use because they can't test it with skills without committing to screwing up their character a little.
I'm not really sure where the best point to have the cutoff would be, but I think having it at least around level L or K would be a good place to start. It's not like the average person can't forget anything, right?