This is because they are adjusted to their habitat and cannot survive in new ones.
Plants in freshwater regions are adjusted to the low salt content and would not be able to survive in areas of high salt concentration such as the ocean. When placed in salt water (hypertonic solution), such plants will be negatively effected. The high amount of salt outside the cell will cause water to flow from inside the plant cells to outside. Loss of water from inside the cell causes the cell membrane to pull away from the cell wall, a process called plasmolysis.
When a saltwater plant is placed in fresh water (hypotoic solution), water would rush into the plant cells to maintain an isotonic solution. This would cause the cell membrane to press up against the cell wall very hard, increasing turgor pressure. If the cell is left in this state for too long, cytolysis can occur.
This explains why salt water plant and fresh water plants are found dead when their habitats have been interchanged.