He has gone to eat at the house of a sinner. This was not the first time, nor was it going to be the last time when Jesus is accused of breaching the laid out religious and social norms. However, unlike the other instances, this particular one was unique because in it we find the crowd joining the religious elders in expressing their disapproval. This is because Zacchaeus was no ordinary sinner. As a tax collector, he was a betrayer of his people: he was colluding with the foreign Roman government to oppress his own people. His sin amounted to treason, and he was irredeemable in the eyes of his community. He had accepted to work for a people who were oppressing his own community, and by doing so, he had, in a way, renounced his citizenship. His actions placed him outside the community's salvation 'zone' (excommunication). Moreover, as he later confessed, he used to tax more than was mandated by the law and used the 'surplus' to enrich himself. He was corrupt.
The crowd's disapproval of Jesus was twofold: not only did he keep the company of Zacchaeus, a known sinner and betrayer of his people, but he also 'joined hands' with him at dinner. Meal times were sacred moments in Jesus' community, and only those who were at peace with one another (seeing eye to eye) could eat from the same table (dipping their hands in the same dish). By dining at the house of Zacchaeus, Jesus was demonstrating that he had no beef with him and his lifestyle. As a matter of fact, his act of eating with Zacchaeus could be interpreted as his tacit approval of Zacchaeus' 'sinful' lifestyle. 
In his response to the crowd's criticism of his move to embrace a known sinner, Jesus pointed out that he too acknowledged that Zacchaeus was a sinner. However, unlike the crowd, he believed that all was not lost for Zacchaeus. As sinful as he was thought to be, he was still redeemable. Indeed in Jesus' move to invite himself into Zacchaeus house, salvation had knocked on his doors.
Jesus was not offering Zacchaeus a free pass. He was not asking the crowd to simply forget and sweep under the carpet all the ills which Zacchaeus might have perpetrated. What Jesus was doing was to offer Zacchaeus a chance to right the wrongs he had done. At least he deserved that much, for he too was a descendant of Abraham. Zacchaeus on his part accepted the offer of Jesus. He acknowledged his sinfulness and agreed to begin his repentance by giving back what he had wrongfully acquired. And as Jesus was used to doing in such occasions, he joyfully welcomed Zacchaeus' act of contrition since he (Jesus) knew that he had accomplished his mission.
Jesus' mission is that of effecting reconciliation, that is, bringing back into one what had disintegrated. Through his sinful actions, Zacchaeus had drifted off and had broken away from the community. By sinning, he had broken the bonds that existed between him and God, and between him and his brothers and sisters. By sinning, he had cut ties with his people and put himself 'outside' salvation since his salvation and destiny was tied to that of his community. It is for this reason why it was important for Jesus that this man be offered the opportunity to repent and come back to the community. 'The Son of Man came to seek and to bring back into the fold those who have lost their ways.' For as long as individuals like Zacchaeus remain apart from the community, the mission of Jesus remains far from attaining its culmination.