Is it true that in this situation an invitation letter is a must?

My brother is travelling to New Zealand via China. Hi is not staying there at all, he is traveling via airport and he connection flights are less then 8 hrs in between. Chineese embassy is requesting an invitation letter. Is it nessessary? As my brother has a proof of travel details as well as Visa to visit New Zealand has been issued to him as well.
The embassy is requesting an original letter - not faxed, or emailed. Since when the fax or email became illigal?