All the existing math functions are global. If you add a new one, it's only fitting that it is global too. Having one and only one math function namespaced would be inconsistent.
Not to mention, if they namespaced all math functions it'd break backward compatibility badly. Given that no core PHP functions are namespaced, I don't see why they should start now.
Also, with the functions in the global namespace you can implement a namespaced copy that takes precedence within the namespace and its children. It can easily confuse people, but I've seen it used to force people away from string functions to their multibyte equivalents (it threw exceptions).
