Play with every ingame name in singleplayer!

With this bug you can play with every ingame name you want (only singleplayer).

What I expected to happen was...:
I expected that the minecraft launcher will crash and I must relog with my account.

What actually happened was...:
You can play with every name in singleplayer and I think that is a bug.

Steps to Reproduce:
1. Press "Windows Key " + "R".
2. Than type %appdata%/.minecraft.
3. After that open "launcher_profiles.json".
4. Go to "displayName": and change your name.
5. Now save the file and start the minecraft launcher.
6. Now you will see that your name has been changed.
7.After minecraft has started, open a singleplayer world.
8. If you type something in the chat, you will see that your name has been changed ingame.
9. It doesn't work in multiplayermode, only in singleplayermode.