Clojurians Log v2

The task specification says that "A call to any of both continuations notify termination to the caller, and the behavior of subsequent calls is undefined." I was curious, is there a rationale for making multiple calls undefined behavior instead of simply forbidden? In contrast, the flow specification says "must not be called", "called exactly once", etc. (If it doesn't make a difference, i.e. "don't make subsequent calls to the continuations because the behavior is undefined", I like the "must" style of wording it as being a bit more clear and straightforward).