The dirtiest plants I've seen were in indoor gardens. That said, this was due to owner error. Bathing(the plants, not the grower) is important when growing Cannabis. The plants love being clean. Avoid strong streams of water, but rinse the leaves until they drip, about once a week. Clear water. Indoor plants often appear shiny, like a field worker who hasn't bathed in a week.
I do this outdoors at least once a week, until harvest, taking care to do it in the morning, late in budding to prevent mold.
Truthfully, I suspect most commercial indoor growers would prefer to not rinse the plants because this costs them a little weight. Sometimes I'm pretty cynical. I've seen people pull some underhanded stuff in the name of money.
Too many younger pot heads associate a coughing fit with quality. They like the rush offered by hyperventilation. Dirt seems to amplify that quality. I'm mystified by this.
VG