"Buying the belts" is really just buying the Skycouch product. They don't supply the belts if you haven't paid for the Skycouch product. The one exception is when the Skycouch is given as a HVC benefit - I have had this occur at check-in and a belt was provided by the crew and have seen other comments from people on here who have also had belt kits provided in such a situation.

At the end of the day you're trying to benefit from the Skycouch without wanting to pay the fee, and that's really what NZ are trying to prevent.

I'm assuming United don't sell the upgrade to a Skycouch as an option because it's not actually a different product or booking class it's merely a fare basis change.