You're making a lot of assumptions we don't know.
If Soldiers aren't allowed near farmland, but farmers work the feilds, how do they do them any good?
Vistim, is there once a year, Shin shun outsiders to an enourmous degree, and are even capable of entire isolation for over a year, they do not rely on trade, or even encourage it. A close real world example would be pre-Opium war China, closed off except for a few cities, and barely trading at all.
Szeth is not an exception, the weilders of the Honorblades are chosen, it doesn't happen at random, and are kept close by the Stone Shamans.