The purpose of a democratic society is where debate reaches a point where the best decision for the majority is chosen. It would not be a democratic society if everyone believed the same thing.
The term cult, is another term for religion, but the less popular religion, so in NZ's case Rugby League would be a cult. I think you are wrong to call Rugby Union or the All Black a cult, but you could if you wish to show it in a negative light.
You can argue that the media/political actors are pushing a hegemonic ideal, but that is a very far fetched argument. They are just aligning themselves with the most popular 'brand' (The All Blacks) in an attempt to boost their own brand/sell more. They are not 'forcing' us to believe what they believe.
We (New Zealand) do give credit to other sports, such as the America's Cup, remember the parades down Queen St in Auckland when we won, and the Black Caps doing the unimaginable in the Cricket World Cup and beating South Africa to reach the final. We also praise the less popular sports that we are elite at, including the rowing/kayaking (numerous world titles and gold medals), motorsports (Scott Dixon multiple Indy Car Champion, McLaren is a world famous name in Formula one) etc, but they will never reach the heights of the All Blacks because rugby is our national game. This game is often referred to as a religion in New Zealand, so it is only natural that companies and organisation would want to align themselves with our national religion to make money off it. And the media are only feeding us what we have an appetite from, as they need to sell papers and adverts to stay in business.
Everything else takes a back seat during the world cup. It is the event that defines our country for the next four years. Being the best team in the world, with our outstanding record over the past decade means something less if you are not world champions. So no one will really care about the flag for the next 7 weeks