Umm...this was already touted as possible even before 4e was released. The idea was to have a fighter nab scorching burst as an encounter power (either via half-elf or the wizard multiclass feat), letting him mark everyone in a 3x3 square 1/encounter.What next? Dragonborn fighters multi-classing into wizard to get multi-target marking capabilities five or so times an encounter?

I think the problem here stems from your possibly limiting take of just what a combat challenge entails, which closes your mind to other possible and equally viable interpretations of what it lets you do.By virtue of his role, the fighter is a melee based character. It makes a certain sense that he can 'mark' adjacent targets because he can interfere with their attacks against targets other than himself and if they move, he is in range of affecting them immediately.
My view on combat challenge is that it involves the fighter keeping a watchful eye on every foe he has attacked. The enemy is aware of this as well, and this causes him to be partly distracted in his attacks. He is mindful that whatever he is doing, the fighter is out there stalking him, waiting for the right moment when his guard is down so he can strike. This makes him instinctively become more cautious in anticipation of an attack from the fighter, which in turn hinders the efficacy with which he can attack other players, because he cannot fully focus on this task. In a way, it is somewhat like an intimidation-based effect.
Normally, this would apply only to foes the fighter threatens as he is a melee PC by default. But I see no reason why he would not be able to mark foes at range, be it via archery (say he multiclassed into a ranger) or magic (wizard attacks). So long as he can make his presence felt on the battlefield, why not?
Thus, I disagree that your interpretation is neither necessarily in the "spirit of the rules" nor "wrought of common sense". Likewise, I do not see anything problematic with this combination, so I feel no need to houserule anything. Let the fighter area-mark by all means. He is a defender - it is what he is supposed to be doing, enticing the foes to want to attack him over anyone else.
