Hi Llyn,
Thank you for your question.
It is going to be a funny answer. The host that requested the passport information rejected my booking. We booked with a different host and they do not have the same requirement (he didn't even ask) and we stay with the new host with no issue.
We have 2 families going at the same time. The 2nd family, their host in Osaka initially ask for the passport information as well, but after a few email back and forth, once again, their host ended up not needing that information and the 2nd family stayed with their host. It seems to me this is not edged in stone, or perhaps different hosts have different attitude towards this requirement... I don't know.
I would like to elaborate a bit more here though. The culture of providing identification in Asian countries seems to me, is a bit more of a common practice when compared to North America (not sure if you are from NA or not, didn't look at your profile). When I was in Nagoya staying in a hotel (an institution) and it is asking for passport copies as well. If the government view AirBnB as a hotel business, then the passport requirement is going to stay with AirBnB for Japanese host, I think...just my 2 cents. Whether or not it is being enforced, it is up to the host.
Any question, let me know... thanks
Vince