Boros Aggro is almost entirely red and white threats like Sarkhan, Dragon Ascendant, Magmatic Hellkite, and Nova Hellkite, making Flashfreeze a clean, efficient counter against their red spells. Day of Black Sun sweeps their low-curve aggressive creatures (most are mana value 4 or less) before they can race us. We Say Thee Nay! is too slow against a fast aggro deck that plays its threats early, and Azure Beastbinder's small body and conditional attack trigger can't keep pace, so both trim for cheap interaction and the sweeper.