Username or EmailPassword
....I don't think they chose to have an unsecure system. Who would?
I think, in a way, that they did. Windows networking is essentially promiscuous (this is not meant to be derogatory, but I cannot think of a better descriptive word) in that it, by design, will talk to anybody, easily.
The upside is that networking is relatively easy to set up, in accordance with MS general concept. The downside is that it tends to weaken security.