score:4
This is not addressed in the Preface to the 1984 NIV, so we must look elsewhere for clues.
The best explanation I've found comes from Craig S. Keener's commentary on Matthew:
Some take sinners here to mean the 'am hΔ'Δres common people whom the Pharisees despised for their lack of adherence to Pharisaic food laws (as in Jeremias 1972:132; thus the quotation marks in the NIV); more scholars today lean toward the view that it means sinners in a more blatant sense. (source)
So the NIV translators make a judgment call β when the word sinners is used to mean something more like "despised by Pharisees" than "those who violate God's law," they put the word in quotes.
James A. Brooks, in The New American Commentary, explains the matter similarly, regarding Mark 2:15:
The NIV is quite correct to put the word "sinners" in quotation marks to indicate that it is being used with an unusual meaning. The reference is not to immoral or irreligious person but to those who because of the necessity of spending all their time earning a bare subsistence were not able to keep the law, especially the oral law, as the scribes thought they should. As a result the scribes despised them. Perhaps a better translation would be "outcasts" (GNB). (source)
But, as Keener notes, this is an interpretive decision and there's some disagreement over it. Thus it shouldn't be surprising that other translations, including the NIV 2011, don't adopt this stylistic approach.