It will mean the end of cheap data SIMs for visitors, and it's going to take away a large source of revenue during the Tokyo Olympics for scrappy MVNOs like JCI. Maybe the Police haven't considered the olympics and the number of visitors that would need mobile data access, or the have and that scares them.
As I have mentioned numerous times (and why I think an unlocked iPhone could have been a game changer), MVNOs usage is exploding in Japan. The idea that "Japanese people don't even know what a SIM card is" outdated and no longer really applicable. True, there are some that of course do not, but if the Metropolitan Police is making an issue of MVNO SIMs, that means a LOT of people are using them.
Anyway, here is the issue. From the Yomiuri:
As the number of people who use a traditional feature phone for voice calls and a smartphone for mail and internet increases, so does demand for data-only SIM cards.On slashdot.jp is the following:
In 2008, a law was enacted requiring identification verification for the sale of voice SIMs, but data-only SIMs are exempt because the Ministry of Infrastructure and Communications didn't imagine they could be used for nefarious purposes since they lacked voice capabilities.
However, downloading free smartphone apps allow voice communications with data-only SIM cards.
Therefore, Tokyo Metropolitan Police Chiefs have pointed out that "as technology has evolved, data-only SIM cards now effectively posses the same functions as voice SIMs. It is necessary to immediately begin identification verification for [data-only SIMs]."
One incident involved the usage of LINE and KakaoTalk to recruit high school girls into prostitution.As I understand it, when Line detects a "44010" SIM*, it requires you to login to your mydocomo page, which users of MVNOs don't have. This means line can't even be used with an MVNO (of course other services can, though).
*440 is Japan's Mobile Country Code and 10 is Docomo's Mobile Network Code. So a docomo SIM is often simply referred to by phone geeks as a 44010 SIM. MVNOs using Docomo's network provide 44010 SIMs (which is why they will be locked out of the Docomo iPhone 5S and 5C).