It's important that a clear definition is given on this kind of thing, so that we understand the reasons why something is allowed and something else isn't. Think of it as two kinds of advantages, basically. We have internal advantages (Mods, hacks) and external advantages (better computers, clearer screens, more accurate mice). It's obviously best that everyone is the most even from the get-go, but that isn't possible with external advantages. For obvious reasons, we cannot moderate external advantages. It's the opposite for internal advantages, as we can monitor these. Some internal advantages (Optifine) replicate an external advantage, thus making the playing field more even. The downside to that is the zoom advantage, which isn't seen as a big enough advantage to give up the benefits of allowing optifine.
The only way fullbright/gamma editing is going to be added, is if it is seen as replicating the external advantage of a brighter screen. At the moment, it isn't seen as a big enough external advantage to need an internal replication, so it is not allowed. I don't see that changing anytime soon.