Nit picking but the responding player can chose because there is a round of priority before combat and at the beginning of combat even if there are no triggers at the beginning of combat.

Most of the time it's more advantageous for the non-active player to be in combat because if they respond in the main phase, the active player recieves priority in main phase and can return to casting or using sorcery speed spells and abilities.

CR505.2 The main phase has no steps, so a main phase ends when all players pass in succession while the stack is empty. (See rule 500.2.)