The BATNA for the employer is training a new person who isn't going to make every 6-12 months a fight.
Suggesting that it's fine for the employee to exercise their BATNA, but that it's not OK for the employer to exercise theirs seems a bit odd.
what, they'll find a better and better job every six months?
From the commenter above, make your employer fight for you every 6-12 months.. A lot of people hate stack ranking ("making your employee fight for their job every 6-12 months"), so why do people think that companies will want to be in the reverse position.
how they'd be paid better somewhere else, deal with it. Denial gets you nowhere.
The above poster said, "wished the employee all the best with his new job.", which is dealing with it, and not denying it. It's a win/win for everyone. The employee gets a new position with presumably higher pay, and the employer gets a chance to hire someone who isn't going to make them "fight for you every 6-12 months".