I've not seen the movie so can't comment on whether it's any good or not, but I can say that Haka *are* unique to New Zealand. Saying it's not exclusive to NZ is like saying the Star Spangled Banner isn't exclusive to the USA - sure it can be performed by others, but it doesn't have the same meaning as it does to those who 'own' it. Haka is the Maori term for a group vocal/dance which can be a greeting, acknowledgement, or challenge ('war dance').
Similar dances are performed by many cultures around the world and, across the Pacific, a number of specific ones are used by rugby teams before international matches. These all have different names, i.e. in Samoa it's the Siva tau, Tonga have the Kailao, Fiji have the Cibi. Ka Mate is the Maori haka made famous by the All Blacks and is considered intellectual property of the Ngati Toa iwi as it was gifted to them by the NZ government very recently (they've been contesting ownership of the dance for some years, tho).
The issue with it being included in this film has nothing to do with the real Highlands rugby team performing Ka Mate - that's a separate issue. The problem was the makers of the film not asking permission to use it in the film and making money from it - much like any recording artist would expect to be asked/remunerated if their song was part of the soundtrack to a film.
Personally, I feel that if a team wishes to perform another culture's dance they should at least understand where it comes from, what it means, and faithfully reproduce it - preferably with permission. Otherwise - feel free to come up with your own! Make something original and special to your own team and circumstances.
