The cleave doesn't care about AC of the secondary party just like the other ones that provide damage on miss don't care.
It works insofar as it seems the common thread of 4th edition design is to take into account that hit points aren't actually for sustaining damage.
My guess is that you aren't supposed to take out minions with this power either, just like you can't kill them on a miss (which is in effect what this power provides.)
The visual probably isn't that you cut through your foe and strike the secondary, but you sweep your blade through your opponent into adjacent areas, so the adjacent foes are using up precious (luck) hit points in dodging just like they would have if they were the direct target.
It works insofar as it seems the common thread of 4th edition design is to take into account that hit points aren't actually for sustaining damage.
My guess is that you aren't supposed to take out minions with this power either, just like you can't kill them on a miss (which is in effect what this power provides.)
The visual probably isn't that you cut through your foe and strike the secondary, but you sweep your blade through your opponent into adjacent areas, so the adjacent foes are using up precious (luck) hit points in dodging just like they would have if they were the direct target.