The one argument I get sick of hearing bandied around in cases like these is: "but those companies invested a lot of money in training their employees. How would you like it if you owned a business and trained someone only to have them poached?"

In the words of Dwight Shrute: "False!". Most of the employees in question in this case and probably every other case where poaching is worthwhile are autodidacts. These employees are valuable precisely because they work hard to increase their own value. They know that, and they deserve to be paid accordingly by shopping themselves around.

It is in the market for autodidacts where actions like these are most damaging to the individuals affected.