Technically, the team is open to any who have at least 1/64th Maori blood, but realistically anyone who wants to nominate for Maori ancestry can do so (unless they are obviously Pakeha or of Islander origin). It's probably one of those things that have been going on for 100 years and nobody's bothered to question or change it. Kiwis in general don't seem to mind (especially when the Maori win).