Jesus' counsel to "view others as 'a man of the nations and a tax collector'" does not necessarily correlate to what the JW's do.
The whole context (centered on Matt. 18:15-17) is about a personal wrong done toward an individual, not the church or congregation. Jesus said that "you" as the wronged individual can treat the person as a man of the nations. It doesn't mention a congregation sanction here.
Paul specifically talked about how a wrongdoer was to be treated in 1 Corinthinans. Then later, he wrote at 2 Corinthians 2:6 that the "rebuke of the majority" was sufficient to straighten out matters and now it was time to encourage him. Notice the word 'majority'. No top-down order was implied where the elders decided that a man should be treated differently. A 'majority' participated in the rebuke, a minority may not have. No whole-organization shunning is implied here.
Also Paul wrote that those who are mature have, by use, their perceptive powers trained to distinguish right from wrong. So Paul was saying that mature Christians could discern who good associates are and who they may need to steer clear of.
So again, I say that the top-down decision from 'on high' (the elders or the organization) is particular to the way the Watchtower Society wants to run things and is not how things were done (at least in the time of the early Corinthians).