"In most legal systems, confidentiality is part of hiring someone."
I assume you're talking about European systems, in which case it's not true, at least not to a significant extent. And when you hire a contractor it's even less defined - a contractor can freely talk about what kind of work he did for a company, right down into quite specific detail, without a proper NDA or clause to that extent in the contract.
In most legal systems, confidentiality is part of hiring someone. You often sign an NDA nevertheless but its value is mostly declarational.