Problem trying to remove sub-query and use WHERE only to return certain rows
Without a predicate to describe matches between the two tables, you're simply listing all product / Japanese supplier combinations, regardless of whether or not the supplier is the right one for each product.

USE TSQL2012
GO

SELECT  
    p.productid ,
    p.productname ,
    p.unitprice ,
    p.supplierid
FROM    Production.Products p
JOIN  Production.Suppliers s ON p.supplierid = s.supplierid
WHERE   s.country = N'Japan';