Japan's claim to the southern Kuriles goes back to the Edo period 1600 onward. Why would the world treat Russia's claims to them since '45 differently than they treated any other of Russia's bogus territorial claims?
The disputed four southernmost Kuril islands are worthless except for national pride. Russia also has no aims against Japan — If they wanted to conquer any more land than they already have, they’d rather have a warm weather Indian ocean port through Iran, which was their strategic plan in the 18th century. Right now, though, they’d rather get stronger economically.