It is not ignoring the private key, it doesn't know how to sign the transaction because the signature scripts are empty. It also uses the given private key if the command is called using the "privkey" parameter, otherwise it will call the wallet instance and asks if it can sign the given tx.
https://github.com/spesmilo/electrum/blob/64a931f21e6f6699f192158e54ff8f2883682604/electrum/commands.py#L397-L402