Also, bad for the employer that trained you, they may be employers willing to immediately pay you more because you have already been trained and so they don't have to bear that cost.
It is zero-sum when the employee burdens the cost of the training when they leave and makes up the difference in that higher wage in the new job, but that's what is considered a problem here. The suggestion is that a zero-sum situation isn't fair to workers; that the employer should both burden the cost of training and pay a higher wage to those already trained. That is not zero-sum.
It's zero-sum.