Thousands upon thousands of salad leaves are taken to a central plant, washed together, bagged and shipped. Even if only a few leaves are tainted, harmful pathogens can spread in the wash water — the modern salad version of the old adage that one bad apple spoils the whole barrel.
"I would think of it as swimming in a swimming pool in Las Vegas with a thousand people I didn't know,"
Same goes for herbs, as I well know.