Legality of anything from a shell based game engine to a 3D rendering engine are all based upon the terms of the End User License Agreement(EULA, you know the things people never read...
![Rolling Eyes :roll:](./images/smilies/icon_rolleyes.gif)
) This goes with any software. Of course, if there is a clause in the EULA that states the license may be subject to change at anytime, then you would have to worry. If that is the case they could just up and change the license to have all the users pay any kind of fee they want along with anything else they wish.
In short, EULA's define legality.
Think outside the box.
To go on an adventure, one must discard the comforts and safety of the known and trusted.