I think that the meaning of the parable is clear in saying that the justice of God is a generosity which goes beyond what is just and especially surpasses our little ideas of what justice is or should be --- and that is certainly true.  God's love is nothing but generosity; He is always looking for a way to be good to us and creating possibilities where there are none visible to us.

But that can cause a major problem for us when we see innocent children starving, abused, or suffering from cancer.  How can we speak of the love and generosity of God if we can find such cases in our world, and not as rare events? 

There is a response to that question, but it has its own problem.  That response is that we simply have to trust God, absolutely and in every situation; we have to practice the cardinal virtue of hope.  And the related problem is that such an answer seems too simple, that God must be held accountable for an illogical and absurd world (to use the term of the existentialist philosophers), one in which the innocent and the powerless die and the evil prosper. 

In the end it all comes down to one simple fact: God is God and we are not.  If all His actions, every way that He interacts with us, could be explained logically and be beyond reproach He would be only human.  God, by His very nature, is beyond our understanding and is only truly satisfying to us as God, if He surpasses us and is so different than our flawed humanity that He is and always will be mystery.  Any other God, no matter how wise, loving, or attractive, is simply not big enough to answer our longing or to deserve our complete and constant adherence.

Jesus shows us here only one aspect of that God that we believe in, and even in that generous love He is wrapped in mystery.