I know some of the people who campaigned for that. It's not just 'sign language' - it's New Zealand Sign Language (NZSL), which is native to NZ and evolved through several hundred years of the deaf community there.
Calling it 'sign language' is like saying 'we reconise spoken language as a national language'. Yes, but which spoken language?
NZSL is historically related to British Sign Language (BSL) cos of immigration, but it's gone off on its own track and is now a NZ language in its own right.
Are you claiming that NZ should ignore a language that is used by a community across NZ and nowhere else, the only language used fluently by many members of that group, with a rich history of poetry and performance in that language?
Most scandinavian countries reconise their respective native sign languages, the UK has just (sort of) reconised its own BSL as a national language (tho still not with the same status as Welsh, despite being used by far more people than Welsh)
Anyway, returning you to your bickering over spelling - see my next post below.