Showing Gratitude to the Swindlers

The Gemora (Kesuvos 68) cites an incident: Rabbi Chanina had a poor man to whom he regularly sent four zuz on every Erev Shabbos. One day he sent the money with his wife who came back and told him that the man was in no need of charity. Rabbi Chanina asked her, “What did you see?” She replied, “I heard that he was asked, ‘On what would you like to dine? Would you like silver or gold tablecloths?’” Rabbi Chanina remarked, “It is because of such cases that Rabbi Elozar said: Come and let us be grateful to the swindlers (who pretend to be poor), for were it not for them, we would have been sinning every day (on account of ignoring the poor).

*** The Gemora cites a Scriptural verse proving that one is regarded as a sinner if he does not give charity to a poor person. Why is a special verse necessary? The mitzvah of giving tzedakah should be like any other mitzvah, and one who refrains from giving to a pauper should automatically be transgressing this mitzvah?

*** How can we give gratitude to the swindlers; it is prohibited to provide assistance for one who is committing a transgression? Cheaters and swindlers are from the group that will not merit seeing the Shechina; wouldn’t it be better if there weren’t any swindlers at all?

*** What sin are we showing gratitude to the swindlers for? Is it for the sin of ignoring a genuine poor person, or perhaps, it is only if we ignore a person whom we are uncertain if he is actually poor or not?