Why does the Torah discuss the burning of the entire land?
Yerushalmi Kil'ayim, 9:3: To teach us that when Eretz Yisrael burns, it serves as a Kaparah for the biggest Resh'im 1 - even those of the caliber of Yerav'am ben N'vat and is henchmen. 2
Perhaps the Yerushalmi learns it from the word "Kol Artzah"?
See Torah Temimah, note 18.