We wish it were so simple. If a candidate responds to an ad, that's not piracy. If a Recruiter contacts a candidate directly, that is. An ad is a communication. No more, no less.
Websites are ads.
Telephone calls are ads. Email is an ad. Intellectually, we are tempted to say that all public communication has some advertising component. If you want to establish a policy of never Recruiting a currently employed professional, we can understand why. A policy that limits the labor source to the currently unemployed is a recipe for disaster.