You need gas to post your "Release" to Ethereum blockchain and gas costs money. How is it possible to be free?
“Any message, regardless of its original size, only generates a 32 byte content hash and costs about $0.03 to submit to the blockchain.”
Also
“CrypoProof is free to use because instead of sending each content hash as a separate transaction, CryptoProof combines all the hashes posted during the same hour into a Release. The release simply contains all the content hashes posted that hour.”