That power would trigger a mark, just like Twin Strike would if you split the attacks. No confusion about that, that is the RAW, it says it under melee attacks, specifically declaring multiple attack rolls = multiple attacks for melee. The full definition of "attack" in 4e, according to pages 269-276 of the PHB and Monster Manual 1-3, is you roll a d20 to see if you hit AC, Fort, Reflex, or Will.
As has probably been noted, several things are designed and worded by people not familiar with this definition. Doesn't change the printed rules.
Except that would rule out close and area attacks because they too only make one attack roll per target, and you don't make the same attack roll for two different targets.
If you have two interpretations of the rules, and one leads to utter nonsense, you go with the one that makes sense. Because the other is obviously incorrect.
The marked condition cares about the Target: line, not the attack: line or the Hit: line. If the fighter is included in the Target: line (or the equivalent for monsters) then the mark does not trigger. It's that simple.
In the case of Twin Strike, it has one target line. So therefore, if the marker is one of those target, the mark does not work.
In the case of a 'make three claw attacks' powers, it's -three- target lines. You're not executing a power that says 'Target: one, two, or three creatures.' You're executing a power that says 'Use the Magic Missile power three times' or 'Use Eldritch Strike three times.' The claw attack is a seperate power, and you are using a separate power three times.
An alternate theory is that WotC started making more monsters with melee/ranged multi-attacks because a common complaint from players is that their DM's never violate their defender's marks. These powers would give the DM a good incentive to violate the mark(compared to a single attack power) without forcing them to do so.
This.
Defenders are not ubertanks who are supposed to take all the attacks ever. They are to take a proportionately higher number of attacks, but not all of them. This isn't WoW where the defender is tanking five huge internet dragons and is expected to take more incoming damage than the apocolypse. This is D&D, where your other guys are gonna take hits now and then.
I think many people in this thread simply have too much of an expectation that tanks be able to 'tank it all.'