It has built-in sharing functions (e.g. Twitter vs Email vs ...), but it doesn't have Share, or any other general sharing extension point. This is why some iOS fans think Twitter integration is a big deal (and why most Android fans are very confused when they hear that).
I'm not sure whether the restriction is design-driven or business-driven (or both). It may just be an expression of Apple's preference for minimizing options and customizations. Alternatively, it might be that Apple wants to control the sharing menu "real estate" and require Google, Facebook and others to make individual deals to get a spot. Twitter might know (since they should have first-hand experience with the integration interface).
