The 2-for-1 deals mean you get 1 for free so if (either) one breaks you cannot claim it under warranty as it was not sold to you, it was a free gift with zero value. It is a way of selling product with (even) less quality control.
Wouldn't this work the other way in NZ? That is, without proof either way, the consumer would be given the benefit of the doubt over which was the gift and which was bought? In any case, CGA covers free gifts as well.