Considering for a moment...

...that GD is clearly the Prisoner's Island of Boards, where toxic players go to languish, and Riot is ok with this; why again is it that they do not apply a design concept they clearly enjoy the success of to their actual game?

The answer I believe is simple. The only logical conclusion that can be drawn is that Riot's Elite Inner Circle is in fact comprised of emotional succubi who feed on the suffering of the playerbase. Our tears nourish them. Illuminati Confirmed.

But seriously I think Riot's reasoning behind not implementing a Prisoner's Island system was a blatant lie and they in fact will never implement such a system because a larger playerbase is more profitable to them than a happy one. It wouldn't behoove them for the griefers to go somewhere else.