Koha has been trademarked in various contexts in NZ already. We thought the same as you, but no, It can be trademarked.
A selection of Koha trademarks in NZ
Bell tea & coffee hold a Trademark on Koha - in relation to tea; coffee; tea and coffee substitutes; preparations for beverages
Tohu Wines have Koha, in relation to alcoholic beverages (except beers); wines; fortified wines
KOHA PRODUCTS N Z LTD have a TM on Koha in relation to cosmetic preparations; soaps; perfumes; lotions; face creams; eye creams; make-up; hair care preparations; toiletries; essential oils for cosmetic and perfumery use; essential oils for personal use
I agree with you that it shouldn't be trademarkable, but it is.