Re the whole discussion on this thread, you could also argue that discriminating against the disabled, the elderly, or women in general is economically rational, but it's still illegal. I for one am glad for that.
However, it is NOT illegal to choose not to hire someone, or to fire someone, because that person does not meet the current requirements for the position. If one of those requirements is being available to work for the next 12 months, the employer is free to choose based on that. The same does go for the sick. I cannot fire you because you have cancer. But I can fire you because you have not been able to report to work for four months. Again, if the basis for the hiring/firing decision is availability to work, there is no wrong- either legal or ethical.
For example, I cannot fire a current employee because she tells me she is pregnant. That is illegal. I cannot deny a promotion or other employment benefit on that basis either. However, if work absences related to her pregnancy cause her to miss out on an experience/project that is a prerequisite for that promotion, I am not obligated to give it to her. Similarly, if I hire someone who then asks me for six weeks of unpaid leave four months after the hire, I can legally deny that leave. I can also legally fire her for not showing up.