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]).
IANAL, so I won't quote relevant text from previous link, but I believe promotional photos need to be of the actual food (ie not substituting shaving cream for whipped cream to deal with melting issue), but it will likely be a very picturesque version of it, though they are indeed (less, but still) askew in those photos. I looked a bit further, and it seem the American BK uses similar (though not exact) photos of their JP counterpart now, maybe it's a trend, but I did find a rather stark contrast with their AU counterpart (naming is another story):
[0] https://www.japaneselawtranslation.go.jp/en/laws/view/2303/e...