Here's what the protests were about: In April seven people were arrested on charges of "disrupting official duties" as they were protesting against the fact that homeless people in Japan are not allowed to vote. To vote in Japan you need a certificate of residency (jûminhyô), which in turn requires the possession of an address. In addition to being poor and lacking a place to live, the homeless are thus prevented from exercising basic democratic rights.
The resident's card system is also hugely discriminatory against the homeless for many other reasons: without it it is impossible to get an ID card, a bank account, an insurance or a mobile phone - de facto preventing the homeless from receiving welfare or pensions or applying for a regular job.
Additional info (in Japanese) about the demonstration can be found here.
To deny people basic democratic rights for lack of a certificate of residency is absurd. It is also absurd to maintain a system that exacerbates exclusion. I give all my support to those who campaign for everybody's right to vote as well as to the demand that the arrested be released immediately.