You can change the read, write, or disk cache policy of a virtual disk.
Read ahead and write back requires cache. Therefore, if the controller does not have cache, it does not allow you to set the policy value.
Similarly, if the PERC has cache but not battery and the policy is set that requires accessing cache, then data loss may occur if base of power off. So few PERCs may not allow that policy.
Therefore, depending upon the PERC, the policy value is set.