Since the very beginning the AP manager on a Cisco WLC would never respond to pings. Well that has all changed if you use LAG and an AP manager with 7.x code!
I like how Cisco hides little nuggets in their documentation. It states, in LAG mode, the management and AP manager uses the same base LAG MAC address.
Note 
With the 7.0 release onwards, the MAC address of the management interface and the AP-manager interface is the same as the base LAG MAC address.
LAB
A show ARP on the distribution switch you can see the MAC is identical for both the manager and AP manager.
NOTE --
This was tested on 4402,4404 and 5508 model controllers.
AP manager(s) aren't needed with a 5508.
This only applies to a WLC in LAG mode w/ AP Manager
Additional Reading Material:
http://www.cisco.com/en/US/docs/wireless/controller/7.0/configuration/guide/c70mint.html#wp1117168