There seems to be some odd ideas as to what an AROS ROM is... All Amigas need Kickstart software to boot, AROS aims to offer a free open source alternative to the copyrighted Kickstart software produced by Commodore and licenced by various companies since.
Apart from the A1000, the Kickstart software for the Amiga was stored on ROM and thus has always been refered to as the "ROM"...
As a classic Amiga user, AROS Kickstart software offers you an updated Kickstart that may include features unavailable (built in RTG for example) in the original software.