1) Yes, it is illegal to harvest or cut any tree without the landowner's consent. Consent must be via a signed timber sale contract with a licensed timber buyer.
2) No, it is not illegal to harvest black walnut trees; unless, of course, these trees were illegally harvested (i.e., harvested without the landowner's permission).
Please contact an Iowa DNR forester for additional answers you may have:
http://www.iowadnr.gov/Environment/Forestry/ForestryLandownerAssistance/DistrictForesterContacts.aspx