Can play full game in offline mode even without buying it.

Put the summary of the bug you're having here

What I expected to happen was...:
I downloaded the client from the minecraft site to run it on demo mode ( i have not bought the game ) but when loggin in it said "Login failed" and the button play offline appeared. Now i see that i can type in every random username and password and i always get the same result: "login failed" and clicking "play offline" i can enjoy the full game and not just a demo mode... So actually i can play it like for free...

Steps to Reproduce:
1. Download the client from the site
2. get the "login failed" (i dunno why i cant log in from the client )
3. click play offline.

I wrote this here because i couldnt find a "proper" place to post it and being able to play minecraft for free is something that i think was worth a check from you.
I hope this helps you guys.
Ty
Denis