Why not? What an employee does in their time is their business. Furthermore it takes two to tango, it's a free market.

If an Apple employee is solicited and accepts, then they're finding the opportunity or terms more agreeable than whatever Apple currently provides, why should an employee be denied the opportunity?

I trade my time, skills and knowledge for money. I don't owe my employer anything other than the above. If I receive a better offer, why shouldn't I take it?