How is it you figure that this warrants any sort of investigation exactly? I don't see any other issues with this neither: it applies for new signups, existing users aren't required to open G+ account or anything else.
It's funny how people keep getting "shocked and surprised" by obvious moves, what use is it having a profile or a cohesive identity system for all your services if you don't incorporate it into said services or even use it when users sign up?!