What is ironic in the words used by the narrator to describe the summoner in “the prologue” to the canterbury tales?

The Summoner was a church official who was responsible for calling out sinners before the courts. Although this was his job, he himself is a sinner. The Summoner would allow a sinner to keep a mistress for an entire year just in return for a quart of wine. He is sympathetic to such people because in all likelihood he commits the same sin himself.