While you were undoubtedly in the wrong for leaving a magazine in the gun, the responsibility for anyone being hurt would always be with the person who pulled the trigger. Standard rules of firearms apply, always assume a gun is loaded until you have absolutely proved otherwise. Sadly a lot of airsoft players don't heed any kind of caution when handling their toys.
Please don't take this as a dig, clearly you won't make the mistake again! This scenario is a good example of why some rules which seem like they might not apply to everyone really do apply to everyone. If the site rules state that you must clear your gun and remove magazines, it doesn't matter how safe you are... there's always the lowest common denominator that can f*ck everyone's day up