Different plants have different degrees of tolerance for salt. Quite a high concentration of salt is required for it to function as a "herbicide", and there is no doubt that many plants are not adapted to live and thrive in water (whether fresh or salty). Sea plants e.g. "seaweed" are adapted to comparatively high levels of salt, and total immersion in salty water. This is no different really to the case with us humans - your body can tolerate low levels of salt in your food, but it is recommended that you do not ingest more than 6 grams of salt per day. If you were to try drinking only saturated salt solution, rather than fresh water (or other safe drinks), it would not be long before it killed you. But that does not stop you sprinkling a bit of salt on your food if you feel like it. For both plants and humans, salt is only dangerous if present in too high a dose.