Earlier quoted context omitted.
The next time you try to enter or leave Japan after staying for 90 days, they would ask you how you are able to stay 3 months without working. If you cannot show significant savings, then you would get deported and banned for 10 years. Happens all the time.
Does it really happen all the time? Any references to this happening all the time? Because to me it seems like border patrol doesn't have the authority to check your bank account or demand that you "show significant savings". Sure they can refuse to let you in but does it really happen all the time?
The UK requires some visitors to show bank statements when applying for a visa.