It's not about being equal. If everything were equal, chance would determine the best. Fairness when it comes to games lays soley in the rules of the game itself, such that each competitor abides by the same measures of ability. In football, for example, this means a field that is generally the same on both halves, two full teams, and the same rules abided by each.
Supplements are merely one aspect, and the importance of it can vary. An extremely gifted player with a bad trainer could arguably perform worse than an average player with a great trainer. There are alot of factors that lead people to be unequal, supplementation just seems to be the easiest one to single out and test for.
Edit: By my reasoning, different weight classes (when they exist) are completely different competitions. If you won a tournament in the heavyweight division, did you not place first at the tournament? Those other guys that placed first did so in different events. Sometimes, in the interest of having a competition despite few participants, weight classes will be merged. Doesn't seem to have fairness in mind.