I'm gonna assume that we're mainly talking about working in California here. Isn't that an at-will state? If an employee can be fired at any time, for any reason (maybe also illegal ones, as long as they are not obvious about it), why shouldn't they have all the mobility that they can get? There needs to be some level of symmetry here. If you have employees that can leave at the drop of the hat, and employees that can fire at the drop of the hat, both have the capacity of misusing it. Corporations could lose a lot of money in training and process secrets, but the employee has a lot less security (sure they might be very sought after workers, but let's not throw principles on the bonfire because of that).

Both sides should certainly strive to be ethical. But no one should be loyal to anyone and just expect that they will hold up that code of honor when they have no obligation to. When we're talking about necessarily more impersonal billion dollar corporations, that is especially true.