|Lion of St. Mark, Piazza San Marco, Venice|
For 20 centuries Christian preachers have been castigating the tax collectors of Jesus' time as bad people--really, really bad people. Evidently, we've had good reason. Tax collectors were the oppressive agents of the over class. They made large, entirely legal profits by taking more from the people than they paid into the government's coffers. OK, they were "sinners," but they were wealthy oppressors, They did the over classes' dirty work for it and, in that sense, made the whole rotten, corrupt Roman system possible. They collected the money that paid the bills of oppression.
Thus, the Pharisees' question in Mark 2:16 is a fair one. Jesus, the exorcist and wisdom teacher, was supposed to be on the side of the people. So, what was he doing making up to the likes of tax collectors? To do so was unpatriotic. It also made him ritually unclean. Now, why would a supposedly pious man do such a thing? They must have suspected that Jesus was finding a way to fund his operation and pay the expenses on his home. His approach and bad attitudes weren't going to get him any funds from the pious wealthy. The poor didn't have much, if any, money. That left the dirty tax collectors as a viable source of funding.
Assuming that Jesus' motivation wasn't pecuniary, as hardly seems likely given the kind of man he was, then we're thrown back on Mark's explanation. Jesus trafficked with these sinners out of a sense of compassion for them. This, in spite of the fact that they were the minions of the over class. Jesus' compassion for the tax collectors provides, possibly, an explanation as to why Jesus openly challenged the pious over class. He did it to try to communicate the gospel to them as well. He apparently thought that he could do that only through daring acts of public compassion and by challenging the Pharisees and others to learn new ways of thinking.