That would be the charitable reason, but I wonder if it could be more related to their laws on accurate representations in ads (Act against Unjustifiable Premiums and Misleading Representations [0]).
[0] https://www.japaneselawtranslation.go.jp/en/laws/view/2303/e...
That was my first thought, too. Never ever looked a burger from any of the well-known chains the way it looks on photos.
That doesn't seem to bother burger king though: https://www.burgerking.co.jp/menu