I have explained the issue several times... Starting out at a higher level defeats the purpose of all levels before that... Why should one class get to start off ahead, when the rest don't? It's illogical, even if you play the "but they are former champions" card or "but you need a 55 character to create one" card.
I have a level 90, does that mean all characters I create should be level 90? No, it doesn't.
No one says that all characters should start at level 90. Nor at a higher level in general. It is only the DK that starts at a higher level, and maybe at some point in the future one or a couple more hero classes that they put into the game. Blizzard themselves said that the reason they do that is that it just doesn't feel heroic for a Death Knight to walk around Elwynn.