Could these be resold accounts by ViralDenki? I've seen such "plans" in the past where people resold lines in a multi-line bundle where the customer actually doesn't own the account and has to go through the reseller third party for any administrative needs. Other times, like with Visible, each person has their own Visible account with direct billing and joins a "family" group which unlocks the multi-line discount.

Looking at Rakuten Mobile (in EN) they do offer a family discount on monthly plans https://network.mobile.rakuten.co.jp..._en_fee_family with bonuses for referrals and don't offer a yearly plan. It allows up to 20 lines per group. The data + receive sms only, no ID validation needed plan, is also monthly and requires online application by Rakuten credit card holder (which would require JP resident ID validation to apply) https://network.mobile.rakuten.co.jp...plan_data-type. It also seems to be eligible for the family discount.

Since the fees are paid to the reseller as an annual plan I imagine the reseller owns the line. It could work for those who want to use the data all in one quick shot or those comfortable with possibly not receiving a year's worth of service & losing the sms number.