As PZ states in his final paragraph of the post:
When people try to argue that science can’t answer “why” questions, what they’re actually saying is that they don’t like the answer they get — there is no why! (emphasis mine) There is no purpose or intent! — and are actually trying to say that the only valid answer they’ll accept is one that names an intelligence and gives it a motive. That is, they want an answer that names a god as an ultimate cause, and a description that doesn’t include agency doesn’t meet their presuppositions.