How can there be privilege if there's no contract, retainer or payment???
Simple. The relationship exists, with or without a written contract. And with or without the exchange of consideration.
At least in California, a contract for services can be verbal. And there are many implied aspects of contracts.
Likewise a professional owes his client certain standards of performance, ethics, honest dealing, fiduciary responsibility, if a deal and money result, or if they get no result.
Each profession has its own professional standards, plus the state has even more.
Remember how CPAs used to "self-regulate," until Arthur Andersen and Enron?