Who said it was? Do you honestly think though that world first guilds only used RMT once tokens were created? And that they at no time used a third party seller? That is what legal and illegal in the context of this discussion is referring to. It is not touching any ethical game development philosophy.
- - - Updated - - -
Everyone has, or had, the same ability to get gold to purchase BoE's. World First guilds by virtue of skill, fame, and position in the community have an easier time then the average player. But that will always be the case as long as they remain relevant to their fame. Anyone of skill though can work their way up the ladder to these guilds. Because they are always looking for backups, new blood, or what not. Nothing is also stopping 25 skilled players from starting their own world first contender guild and thus getting to do the same tactics.
You can't have Blizzard run the game on the idea that everyone gets equal gear and rights when individual players are still free to do what they want with the game, or their money. But notably the world first guilds could only buy power when someone else decided to forgoe their right to that power. Because tokens sell gold not items. A BoE can only be bought when another player decides to sell it.
And lets face it anyone that isn't in a worlds first guild is likely better off selling it to cash in because they won't really need whatever boost that BoE provides.