There's two aspects to consider here. One is balance; and there's a sense that it's too strong to have marks trigger on each individual melee attack when they're part of a single attack power.
I find that intent to be entirely reasonable, but also to be a terrible line of reasoning to follow. You're not making the game any easier to run by mixing up balance and plain interpretation. As far as I can tell, there's only one way to reasonably interpret RAW; and that's that each individual melee attack
is an attack - it's printed clear as day, and not even in some descriptive text, but in the short definition of a melee attack (and similarly for ranged attacks) Also note that this separation aligns naturally with damage rolls, and works well with interrupts (each attack can be interrupted separately).
Note that
there is no meaningful ambiguity conerning the word attack in this context: You may well consider an attack power an attack, and additionally or instead of that consider a melee "multiattack" an attack;
it doesn't matter for marks! Marks trigger when you make
an attack that excludes the marking creature. Whether or not an attack power is itself an attack or not, and whether or not the group of melee attacks is an attack thus makes no difference at all to the functioning of a mark if the individual melee attacks are
also attacks. The concept of "attack" does not need to be exclusively one of either a power, a group of melee attacks, or each individual attack. It can perfectly consistently (and quite reasonably) be all three. It's just that for the purposes of marking, the only one that matters is the one that makes attack rolls and is the most fine-grained.
Beyond a shadow of a doubt each individual melee attack
is described as an attack; it says so terribly plainly on PHB 270. What
else is an attack doesn't matter!
As a practical matter, many contend that solo's and elites need all the help they can get; and that further, whatever RAW the
monster designers got confused and interpreted it differently. However:
- Nowadays solo's don't need every helping hand they can get; many have anti-marking abilities built in so as to not make this a huge quandary.
- Violating a mark isn't so serious nor a defender so invulnerable that attacking one is terrible as to be a generally big problem. If this discourages you from using poorly designed solo's, even better.
- An ineffective monster isn't a huge problem; a pointless PC role is. A much more common scenario than solos are common monsters and elites; and if the sole requirement is to point one of many multiattacks at the defender, the role becomes largely redundant; a striker would have the same effect but better damage.
The marks
should be annoying to the monster; that's kind of the point. Reinterpreting rules to mitigate PC abilities like that is just poor form.
Unfortunately, you may encounter some monsters where multiple melee and ranged attacks truly need to be considered one to make a monster work properly. Fortunately; you're the DM: no problem! Just reinterpret
those attacks as close and area attacks and there's no problem. That doesn't require you do it as a general rule.
Whatever the RAI; it's extremely annoying that this hasn't been errata'd and made crystal clear. The OP isn't exactly the first person to stumble over this, and it's not like WotC aren't churning out enough errata as it is; and this one would actually be useful... (and if they're reading this, make sure you consider attacks that reference other attacks e.g. "make two claw attacks" and be clear about that case too...)