Why is it not possible to choose the default RX_MODE (without AACK enabled)?

In at86rf2xx_netdev.c:_set(NETOPT_AUTOACK) the comment says /* don't set res to set netdev_ieee802154_t::flags */. However netdev_ieee802154_set never handles this cases which causes ifconfig <n> set -autoack to fail with an error message. What was the idea behind it?