With 802.1x, usernames / passwords or certificates are used. A user's account can be disabled, locked out, or removed, and you'd want to shut off their access.
With PSK, everyone uses the same password. If you were to change the password, you'd have to disconnect everybody from the network anyway, and they'd have to reauthenticate. Can anyone think of why you'd want to set a session timeout on the WLC for a PSK WLAN and force a re-auth at regular intervals?