The colonist were angry with the British for taxing them so much, so they revolted by destroying english tea. In doing so they got access via the ships captain. But, they weren't off the hook as they couldn't do any damage to the ship it self. So, instead of sinking the ship they just through tea overboard.
They didn't care if they were caught because they fighting for their cause. Anger and revenge fueled them so that they could lift such heavy crates. They only broke one thing on the entire ship, a padlock, which they then returned and replaced it with a new one the next day.
But they still had to get out quickly. Though the gauntlet stopped them from stealing and doing any real damage to the ship. However, they were still determined to fault the British Empire's taxation of the colonies. So they did destroy some tea but not the boat.