Malbim: They need to thank Hashem, for the great Chesed that He began to do for them, He did also after they sinned - "va'Yizkor Lahem Briso va'Yinachem k'Rov Chasadav." Hashem is good for His nation and His Chesed is forever, just they did not recognize His Chasadim and angered Him in every generation - "Lecha Hashem ha'Tzedakah v'Lanu Boshes ha'Panim" (Daniel 9:7).