Cant change name when logged in with a mojang acount.

I cant change my name when I'm logged in with my mojang account.

What I expected to happen was...:
I wanted to change my name because i didn't pick the write name i wanted.
And it keeps the name you had when you first logged in with your mojang account.
What actually happened was:
My name was dark and i couldn't change it.

Steps to Reproduce:
1. Sign in with your mojang account.
2. Go to settings to change your name.

You will see...