Employee poaching raises the value of workers. Paying workers less than they are worth is what is truly unethical and all too common among powerful businesses. It is true that the practice of employee poaching has negative effects such as degrading a company's culture of loyalty or cohesiveness but these are outweighed by the fact that employees are finally getting what's due to them.
While from an individuals standpoint, being a turncoat employee could be considered unethical, but from the company perspective it isn't. The fact is that companies exist to make profit, and skilled technicians and other human resources are highly sought after. Silicon Valley is one of the few places where employees are bid for on an increasing return, instead of employees struggling to work for the least pay at highest efficient as has been common in the recession status much of the country endured.