Well, firstly, very little is fair... Many employers will try to get away with as much as possible. The less they spend, the more they make.
If they require this after they have hired you, and won't pay for it, you have to ask your self a couple of questions.
1.Can you take this off of your taxes? If so, then well, you are not really paying for it so it is not so bad.
2. Will this cert make you more valuable, that is, with this cert can you expect a larger salary? If so, will the employer pay you more?
Often times, things like this is the employer trying to get a senior level person while still paying them only a junior salary...
It might not be FAIR, but, well, that business.
Ask your self if it is worth it? If the money is still good, and the boss is kosher, then do it...
If it is just your boss trying to take advantage of you, and you think that you can find better else where, this might be a time to start looking for somewhere else to work, somewhere that will treat you better.
It's less about whether its fair, and more what you are willing to put up with for the job.