If the recruiter is instructed not to advertise the company's openings in strip clubs or porn sites, that isn't illegal or immoral. This is how I view instructing the recruiters not to advertise on their competitor's website or target other company's employees. Perhaps they think the cost of bad reputation or reprisals from those companies was too high. It doesn't have to be a "conspiracy". As I posted, an arxiv article shows that cartels are a natural emergent phenomenon of markets.
As far as marriage - there was a time when people worked decades for a company and earned a pension. These days esp in the tech industry those days are long gone. Poaching is an additional cost of hiring employees. You want to increase this cost - fine, and it will be passed on to the employees and depress their salaries. Your argument that the correlation is the other way is sofar without basis. I am not arguing that it is not illegal - the courts should decide that. I am saying this enforcement goes contrary to the purpse of the law and no one wins.