Sure, if (hardcore PVP) server hosts wanted the option to kick players with too high gamma set in game, I see no harm. It is a form of "cheating" or perhaps an unsportsmanlike act. Players who wish to have higher gamma than usual could play on servers that don't force this setting.
Just keep in mind that there is no way to regulate or know gamma values the client has set from the monitors' OSD, so in the end it's kind of a fruitless effort.
E: Well, to be honest, turning grass off could also be seen as cheating since you could remain hidden in grass on top of a ridgeline and see other people with grass turned up high approaching... But the line has to be drawn somewhere.