from what i have been told it is excepted, correct, behaviour.
support once told me (and it has been repeated here) that pinging an AP isnt the way to determine if it is up.
you should check the controller, either directly or via SNMP, that is the most correct way.
if you really want to work with ping you could check if the traffic indeed exits at the controller. it is not that it is blocked by the AP, it just travels via another route back.