Earlier quoted context omitted.
Point of correction: Unreal Engine is not open source, the source is merely available for perusal. You still need a license for most uses.
It's 2015 and people still don't know the difference between open source and free software? As you describe it, Unreal Engine is open source.
Free Software = source is available and free as in beer and copylefted (you must also release your derivatives as free software). Examples: gcc, mysql, linux, gnome, gimp
Unreal is neither of these. It's not free as in beer nor copylefted. It's just that if you are allowed to use the source if you license the engine.