So the one giving birth gets 22 weeks. The father gets 12 weeks. The mother of adoptive kids gets 12 weeks.
I see no issue here. There are differences between the sexes and catering / supporting those differences doesn't automatically mean discrimination.
Well, the law does recognize that the person who just delivered a baby went through a lot harder time than the one who simply contributed the other half of the genetic material 9 months before.
After my wife gave birth, she breastfed every 3 hours (typical for a newborn) so that means very little sleep. Recovery from having pushed another human being through the birth canal (or from having the abdomen cut wide open) is not trivial.
Then there's the baby's bonding, which is primarily with the mom in the first few months.
Paternity leave is a great thing; as a dad I got to support my wife in child with all the household stuff. And there is not replacement for witnessing the first few months of one's child's life. That goes by so fast.
But as a dad, I recognize that the largest burden was born by my wife. Many trivialize the effort women go through but I think they are the unsung heros of humanity.
And to be fair, it's a hell of a lot harder on a birth mother than anyone else. You'll notice adoptive mothers in this policy only get the 12 week benefit too, so this one is already fairer than most.
Small study: http://bjp.rcpsych.org/content/169/1/30.short Meta study: http://jama.jamanetwork.com/article.aspx?articleid=185905