Cast ironware is very porous. The idea of seasoning is to slowly build up a sealed and slick, non stick surface from the application of oil and the juices from foods cooked ( think of the 'pickling' done to a carbon knife cutting fruit and vegetables.)
By washing with soap, you are continualy reopening the porous iron surface.It's like eating a store bought cherry pie from the aluminum pan vs aplate. You get that metallic taste.
Older and properly seasoned ovens will make far superior tasting dishes than a brand new one.
The heat of cooking kills any nasties, and as I said if you let the seasoning go rancid cier vinegar will zap it and a reheat and recoating sets things straight.
I pack mine in hard paniers when horsepacking. The stoves are filled with cooking supplies and wrapped snug in an old burlap sack.
Some dutch ovens have been handed down for generations having never seen soap and water. The generation manages to produce another generation to pass them down to: Mine did. I have my great-great grandmother's that crossed the palins in a Constoga. It outcooks the ovens I've been seasoning for 20 years now.