Well no I think that is a terrible example? Perhaps the example could be a lot closer if McDonalds had a written note next to the Big Mac combo saying "these patties will be cheeseburger patties, not Big Mac patties" - but if they didn't warn you? That would be entirely unfair and not even close to being an honest comparison.
Mixed class fares are just a way of life. There is literally no way around this. Sometimes J fares simply don't exist the whole way but they give you the option to book it partially. If you book a J fare CHC to LAX, guess what? The CHC flight is, shock horror, in economy, because no J class exists. No one gets up-in-arms about that. Should Air NZ not list that as a business fare because it isn't J all the way through?
I do think that they could be better sign posted. I am not convinced that they're done like this though to get people to accidentally book the airfares. I think that this is done to improve conversions by creating a seamless, pretty experience. The result in the occasional person might miss it - though TBH while I'm not surprised, I've never seen anyone actually miss it before until this thread.