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 NIV is not trying to say the tax collector was proved righteous by his actions. With all due respect, I believe that it's attempting 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 reflexive passive whereas other translations appear to construe the word as a simple passive. A comparison of the verse 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 (i.e. prove themselves righteous). But see BDAG for information on Genesis 44:16 and Luke 18:14.
None of these remarks should be interpreted as me taking issue with the NWT rendering. I'm simply reviewing translational possibilities for this verse.