That's the point, the oil from the seasoning process (or rather, the residue it leaves behind) is not dirt, it's an actual layer of coating - and if you use soap on it you wash away the coating.
No, sorry, if it washes away you have not seasoned it properly - it should be carbonised and bonded (polymerised?) to the actual pan, not just a layer of grease sitting on top.
I have multiple cast iron items in my kitchen that I can wash with washing-up liquid, gently scour with wire wool even (sure, too much vigour would probably start to remove it, or at very least scratch up the surface and make it harder to clean/things stick more) with no problem or loss of seasoning.
