Seems weird to discriminate against adoptive parents... I understand the reasoning but seems weird. It would probably be better to have a medical policy applicable to everyone.
If one stretches such an analogy, one could reason that paid sick leave is discriminating against healthy people. Not many people would agree with that one.
As you've stated, a sensible medical policy for everyone would be good. I would imagine that Google has a one.
So that part is more like medical leave than parental leave (even though you're also parenting at the time).