Facebook's responsibility to not promote the harassment of its users exceeds its responsibility to provide details of why they carried out a request.
Additionally, Facebook had some reason to believe that even if they wanted to doxx that person, disclosing their communication to a third party without that person's own permission may have been a breach of UK/EU data protection laws
Deceased person could have "secret" lover whom he was going to marry. He could have secret son who had more right to decide what to do with the page than anyone else.
He wasn't married to person who harassed Facebook. He could have relations with dozen other women all of whom asked fb to erase all history of it including messages and photos. Grieving person might try to demand restoring profile and giving her access to it so she could identify them and make their life living hell. Yes this might sound farfetched and a little bit insane but stories like this really happen all over the globe thousands time a day.
Its easy to agree with fb here especially because "widow" couldn't possibly do anything good with requested information.
I don't want my scumbag parents or extended family getting information about who is carrying out my affairs from random companies after I die. There's a reason why I'm planning on writing a will to specifically exclude them from any sort of inheritance if my husband and I died together.
Plus these were unmarried partners!! Unlike spouses, unmarried partners have no legal standing as next of kin in most of the world.