Actually, I'm going to go out on a limb here.
Both DC and EM engaged in publicity opportunities with their newborns. They are using those babies to promote themselves.
Therefore matters such as how these babies are fed are of public interest, since both politicians have commented on it.
Neither of these opportunities were spontaneous, they were no doubt discussed endlessly with the Tory/Labour press committees, about how DC and EM could come across as nurturing yet authoritative men.
It was up to both of them to refuse to discuss any aspect of their babies' care and feeding regimes. They did, therefore we can discuss their (partners') decusion.
That I heard EM saying "breast is best" indicates that neither of them, nor their advisors have a clue. Breast is not best. Breast is normal whether you like it or not. It would be nice if they acknowledged this, rather than perpetuate the silly belief that bf and ff are somehow equivalent "choices".
Wouldn't it be great if either partner spoke up and said that she'd wanted to bf but couldn't, and been frank and open about that?