It's stuff like this that reeks of "fragmentation", but a different kind. A problem Google's had for a while now, not that iOS is any better. They make these wonderful apps that work so well together, then you actually try to use them and up pops the caveat. Everything does work wonderfully together, except the way you need to use it (aka normally).
Right now, my only option is to manually add everyone's birthday to my regular calendar as a bunch of events - A) defeating the purpose of having a birthday calendar in the first place and B) making every new contact require importing to the normal calendar individually. How dare I confuse that for an oversight.
I think I'm just pissed off about it because I'm not a fan of doing extra unnecessary work as a result of someone else's incompetence. There's just always something with these things, isn't there?