Umm because the YC partners are smart, helpful and therefore awesome to have as investors in your startup. Why would you want to prevent some of the best angel investors in the business from investing in the best startups?

Since YC itself doesn't do follow-on investments it doesn't have to worry about competing with its own partners (unlike the VC LPs).

This new policy seems to be addressing a specific problem: demo day is meant to be an opportunity for all the presenting startups, not a competition for attention between them. I.e. for YC's sake, it should never hurt a startup to present at demo day, so its a problem if not having a yc partner invest looks like a signal of relative weakness.