I think the point to an FC only game (and, yes, if this belongs on another thread let's do it) is simply to play the characters known and loved from the comics without so many personally-driven influences getting in the way.
In the case of G&M, DC has hundreds of characters across the male, female, and racial spectrum with mantles passed down. These are balanced characters with balanced powersets. Now, I'm not any sort of spokesperson for the game itself, but I can understand the draw of wanting to focus on these characters.
Unfortunately, OCs, especially on supers games, can tend to be an absolute mess. There's often too much OOC influence involved, imbalanced power sets, super weird desires for overpowered power sets, and in a lot of cases the lack of familiarity with an OC versus dozens of established FCs can make it hard to find ways to incorporate these foreign characters into an existing canon.
So, I think the point of barring OCs really isn't to exclude one of the 45,000 demographics, but that there is an established canon with balanced characters that works. The game is about the characters in the comics. So, since it is about the characters in the comics/setting, it's not about the ones that arent.