You may not be aware that in some jurisdictions (Quebec is an example), marriage is not a sufficient legal reason to change your legal name, yet most people informally identify with a spousal name change. It would seem like an overstep for Facebook to regulate against this by policy. That's only one of many, many examples where people shouldn't be forced into using their legal name simply BECAUSE it's their legal name.

The UK, for one, does not have the concept of a 'legal name'. Your name is whatever you choose to call yourself.

Even a deed poll has no official weight, it's just a statement of a name you have chosen. Hence 'poll' instead of 'certificate'.

What is so important about displaying the legal name, especially in cases (like the GP explained) where nearly no-one in ones social environment knows it? If Facebook had legal reasons to know it they still could make displaying it optional.