If you spend $20K on an MBA that I initiated and I quit, totally fair to have an arrangement where I have to pay some of that back.
If you fire me? No obligation.
If you send me to some series of certifications because you want me to have them? Also no obligation.
Lets change fire to lay off. There is a subtle but important difference. If you fire someone it is generally for cause - that is they did something bad. You still want someone doing their job, but because of their bad behavior it can't be them. That is if someone gets a MBA, moves to a management position and starts demanding sex of their underlings, you fire them and demand they pay for the education you got.
By contrast when you lay someone off, you are saying "you were good at your job, but we don't need someone doing your job anymore" (or maybe we can't afford to pay someone to do the job). You lost your job, but there wasn't anything you could have done different if you wanted to stay.
For cases of expensive training it is fair to demand they pay you back if you don't stay for a couple years. We are talking about a full masters at a major university though, not just some random 3 day design patterns class or whatever the hotness of the day is.