"Play offline" button seems to be "play free"

It's not an ingame bug, but a bug in client. When i open Minecraft client and type my credentials in the fields, if i digit a invalid password or account, it shows the button "Play offline". When i choose this options, the game starts, but i am not limited to the "demo" version of the game, but the entire game, like if i bought it.

What I expected to happen was...:
Be redirected to the demo version of the game, because i didn't logged in.

What actually happened was...:
I can play the full version of the game, without pay anything for it.

Steps to Reproduce:
1. Open Minecraft client
2. typo your password, returning a loggin error
3. Click the "Play offline" button