It was evident that they underestimated Erin Campbell's determination, assuming it was just a young girl's whim to show kindness and perhaps flaunt her influence at the banquet. Yet, in truth, she intended to genuinely aid those pitiful souls who shared the faith of Saint Sis.
As for the wagons turning back at the granary doors, Erin did not pursue their little tricks to the end, as long as the grain finally entering the storehouse wasn't adulterated.
The nobles of the Campbell Duchy were unlikely to offend the Campbell family over a bit of money from selling grain. This petty gain was probably their servants' idea.
The nobles might not be entirely unaware of their servants' actions. It's just a mutual understanding, long formed, of turning a blind eye.