How do you know its just one lot doing it? It might have started with one group doing this but you will probably find others have copied them now.

I am still not really sure about this grit weed and the idea people are spraying grit on their crops to bulk them up. That makes no sense to me as it just seems like it would screw up the plants and you would get plenty more growth if you just left them alone and grew it properly anyway.

I reckon this grit is getting on the weed due to way its being moved/smuggled. If the weed were being packed inside bags of sand while the weed was still not cured properly you would end up with grit stuck all over it by the time the weed dried out. This makes more sense to me than messing around with your grow.