Where has this happened? I don't think paid maternity leave is mandatory anywhere in america, but the number of companies offering it has exploded over the last two decades.
Once again, getting past the actual details of this case, it's just a matter of ideological differences, but I'm definitely not of the opinion that unfair labor practices should be ignored because of a possible reductio ad absurdum outcome. It's an argument that's always made about these things.
I wouldn't consider it unfair for either party to be able to ask any questions they want. If everyone can get informed to their satisfaction then their contract is built on a solid foundation. It would be as unfair to the employer to have withheld "Oh btw I might have to just leave in 6 weeks" as it would be to the employee if he was told "Oh btw you might have to just leave in 6 weeks."
Everyone is their own agent and shoud have the ability to freely form agreements which they like, or walk away from ones they don't. Laws which deny you the information to make that decision impede that. People would cry foul if an employer shut up shop all of a sudden because his application for residency got denied and they had no idea that was a possibility when they took the job. Fair is fair.
Aus has mandated maternity leave and there was much rejoicing. Yay, any job I get is going to have to pay me maternity leave! Ok, take a minute until the lightbulb over your head comes on, sweetie. Since the employer knows that too, is that going make it more likely you'll get the job or less? There's not a lot of head scratching required to apprehend the inverse proportion between laws which protect certain people versus those people's desirability. It's an argument that's always made about these things because it's true.