Now that we know that in fact, there was "no overriding legal impediment," that stance looks like it was nothing more than a clumsy effort to appeal to the anti-PC, anti-Brown vote (1).
I've said before that Labour probably need a term out of office to sort themselves out and decide what they stand for. This new piece of chicanery means they should stay out until the old guard who were behind this spineless, cynical gesture, have quit parliament.
1 - "NZ to ratify UN indigenous declaration - Sharples," unattributed story published on the Radio New Zealand News website, 7th of July, 2009. (http://www.radionz.co.nz/news/stories/2009/07/07/1245bb75e887)