Well, they probably do need some sort of reliable witness that you actually are pregnant. (If "being on maternity leave" didn't give you some sort of rights vis-a-vis the organization, you wouldn't bother worrying about that status, you'd just tell them "I'll be back in April!" So they need some evidence that you have the right to be on maternity leave.) Yeah, it's stupid to lump it in with medical conditions, but they've already worked out that form for the doctor to sign, why not just use it for pregnancy as well?

Sadly, if they were good at doing HR, they'd probably go to work for a profit-making organization and get paid 20% more...