Matthew 9:11

‘And when the Pharisees saw it, they said to his disciples, “Why does your Teacher eat with the public servants and sinners?” '

To the Pharisees what Jesus was doing was unpardonable. To them their rituals had become the be all and end all of their lives. And they could not see how Jesus could take the risk of being religiously defiled. To them that was offensive to God because of their perverted ideas about God. So they challenged His disciples as to why Jesus was not more fastidious. Why did He eat with public servants and sinners?

Such meals as this would be held in an open room or courtyard, and anyone could gain access to it, and often observe it from afar. No doubt the Pharisees had sent their spies to keep an eye on what was happening. And when they reported back it was then that the Pharisees approached the disciples about the situation. Or perhaps they had acted as spies themselves, determined to catch Him out. In their eyes a prophet who did not live in accordance with their interpretations of the Law was a scandal.