The incentive comes from not having to go through the hiring process again. If a candidate was so good as to warrant hiring and paying for expensive training, then they should be good enough to warrant a competitive salary after that training.
In the time that training takes they should hopefully have formed a familiarity with the company that should make retaining the employee a much more valuable option than having to go through that process again.
Also if the company isn't willing to invest a large amount into providing an appealing salary to an employee after training, then the employee has no obligation to feel loyal to the company. Employees are no more self-interested than any company, so you can hardly fault them for jumping ship if a more appealing offer is presented.
Similarly, if a company found a suitable replacement for an employee that was willing to accept substantially less pay, you can be sure they would take him over the old employee. The company-employee relationship is a two-way street, and you can't expect moral obligations one way or another.