They know Comey is in serious trouble. I would be fairly certain when the FBI director has confidential conversations with the President of the United States the contents of those conversations are automatically considered classified. Those memo's are also key evidence in this inquiry and were leaked with the admitted goal of causing an special counsel which would harm the president.
The reason they keep saying "unclassified" is because they know those conversations ARE classified and Comey has committed a serious crime.
If Comey thought it was perfectly OK to leak the memos, why did he not just publicly reveal them, himself, instead? There would be no need for this sneaky leak through a third party.