Regardless of whether there's a policy or not, most companies are aware that you're pursuing education/training as a means of advancing your career. If they approve your tuition for reimbursement, they are implying that they'll step up with career, salary, benefits or work environment enhancements that reflect that training. If they don't come up with something, there's nothing wrong with moving on. Besides, if you use sick leave, vacation, dental benefits or the like, you don't expect to have to pay it back. And some companies intentionally provide bigger training allowances as a means of signalling support for employee careers (one company gave me $7k a year). That being said, if there's a repayment policy, you'll want to set money aside if you leave before the time expires. That's why some people seek out signing bonuses.
My company says in its tuition reimbursement policy that they're not
implying that they'll do anything for my advancement. Other companies may make similar statements. However, I see nothing wrong with moving on as long as you pay back the money as you've agreed to.
Actually, the 75% tuition reimbursement is the main reason I'm staying where I am. My salary is pathetic, even for an employer known for underpaying.