How to treat `Promise.reject` as a first-class function?

Why can I do this:

> Promise.reject(3);
< Promise {[[PromiseStatus]]: "rejected", [[PromiseValue]]: 3}

But not this:

> var f = Promise.reject;
< undefined

> f(3)
< VM2366:1 Uncaught TypeError: PromiseReject called on non-object
    at reject (<anonymous>)
    at <anonymous>:1:1