For more on AFB, since the article doesn't describe it or the treatment protocols well - https://beeinformed.org/2013/10/21/american-foulbrood-afb/
Apart from rules particular to NZ, because I do not know them, a key factor is that honey and wax containing AFB spores can transmit AFB to other bees in other colonies. The reason this is important is that beekeepers ( speaking as one ) move equipment between hives to balance their growth and give more space to make honey as needed. This is especially true after the honey harvest, when we empty frames of honey and give the wax honey comb back to the bees for them to get ready for the winter.
That makes the timing of this crucial, as it will be less likely for the beekeeper to reduce the damage if the boxes have been removed for harvest, and disassociated from their hive of origin when AFB is discovered. If you can't say where it came from and where it didn't, then it came from everywhere.
It isn't a fun hand to be dealt, and certainly it is worthy of more visibility and research, but it isn't worth wishing he didn't have to destroy it. Even the treatments available need to be used before adding equipment that will be used for honey, because the honey will shield the spores from the treatment and preserve the transmissibility of foul brood.