Here's why:
* Jinzo's Effect: Jinzo's effect states: "This card cannot be destroyed by card effects. Your opponent cannot activate Trap Cards."
* Trap Monsters: Trap monsters are, by their nature, Trap Cards.
* The Interaction: When Jinzo is summoned, its effect immediately negates all Trap Card effects. This includes the effects of Trap Monsters. Since a Trap Monster's effect cannot activate, it essentially becomes a useless monster, making it vulnerable to destruction by other means.
Therefore, if you have a Trap Monster on the field when Jinzo is summoned, it is not destroyed by Jinzo directly. However, it is effectively useless as its effect is negated and it can be destroyed by other means.