Speaking from memory as it's a little difficult to search for but there's no fully FOSS ac driver available for Linux which might be why the BSDs have none, Linux has open source ac drivers of course but they still require proprietary firmware blobs and the performance is terrible compared to the proprietary drivers (think 350Mb/s with OpenWrt compared to 800Mb/s on stock router firmware). Atheros released fully open blob free 802.11n drivers in 2008 with their ath9k driver, although no other manufacturer followed suit. I've heard that there may be licencing or patent restrictions preventing blob free ac drivers from being released but again it's a little difficult to try and verify this.