Luke 18:14 (NIV) reads: "I tell you that this man, rather than the other, went home justified before God" rather than the wording "justified by God."
But knowledge of the NIV's theological orientation suggests that the NIV is not trying to say the tax collector was proved righteous by his actions. With all due respect, I believe that the NIV is trying to convey the idea that God (as the implicit agent of the verb DEDIKAIWMENOS) justified the tax collector. I also checked my commentary on Luke written by Alfred Plummer and he writes concerning the Greek wording KATEBH hOUTOS DEDIKAIWMENOS: " 'This despised man went down justified in the sight of God,' i.e. 'accounted as righteous, accepted'" (see p. 419).
Studying this verse has also helped me to understand what the NWT is doing in Luke 18:14: it is treating DEDIKAIWMENOS as a passive used reflexively whereas other translations appear to be treating it as a passive simpliciter. The comparison of the passage in Luke with Genesis 44:16 and Revelation 22:11 is interesting. The KJV and the NWT both render Genesis 44:16 in a way that suggests the brothers of Joseph want to know how they might justify themselves or prove themselves righteous. But see BDAG for Genesis 44:16 and Luke 18:14.
I have no real problem with the NWT rendering. However, it helps to review translational possibilities for this verse.