Title is not accurate. They're allowing personal shipments under $100, and the rest can be shipped via their UGX service.
It sounds like JP doesn't want to deal with the customs paperwork at scale (edit: also the deposits).
replies(2):
I'd suggest something like: "Japan Post stops accepting US shipments over $100."
To emphasize that it's not in effect yet and that it's to, not from.
edit: Someone went and reverted it to something less clear than everything else