This verse represents some wonderful repartee between Jesus and his accusers that is lost in translation. His accusers make a play on words accusing his followers of not washing before eating and Jesus turns in back on them. In Greek, this verse doesn't read as though it is a question. Punctuation was not part of the original Greek of the Gospels.

Note that Jesus is not denying his "transgression" but saying that his accusers are doing the same thing. They probably throught that Jesus was describing the washing of hands before meals as a tradition that violated the Law. Jesus's response is consistent with his constant criticism of social norms and traditions taking precedence over true spirit. In the original Greek, Jesus makes it clear that his accusers, the scribes and Pharisees, are more interested in legalisms than the spirit of the law. Much of Christ's teaching in Matthew is aimed at clarifying that a meaningful personal relationship with God and other people are much more important in cycle of spirit than conforming to social pressure and social norms.