Comment # 5 on bug 845319 from Hannes Reinecke

And actually, 'dmraid -p' should fail when there are partitions (or other
device-mapper tables) on that device. The correct action is to remove the
partitions (or other device-mapper devices) _first_ and then remove the dmraid
device.

Much like you'd do with LVM; imagine you'd have a LVM setup on the dmraid
device.
You surely do _not_ want dmraid to succeed here.

(For the same reason the udev rule wouldn't work, neither; we're only getting a
'remove' event if the device has been removed, which it can't, as it's being
held busy by the devices stacked on top.)

So from my POV the system works as designed.