Here's why:
* Effect Trigger: Sonic Jammer's effect only activates when it is flipped face-up.
* Turn-Based Effect: The effect specifically states "during this turn," meaning its effect is limited to the current player's turn.
* Opponent's Turn: Once your turn ends, the effect of Sonic Jammer ends as well, and your opponent can freely summon monsters.
In summary: Sonic Jammer's effect is a temporary effect that lasts only for the turn it's flipped face-up. It does not extend into your opponent's turn.