What's the benefit of prohibiting the use of techniques/language constructs that have not been taught?
Rules like this are generally instituted because the teacher is attempting to teach a concept made moot by one of these constructs. For example, as a teacher, if you're teaching bitwise operators and ask students to implement absolute value, it simply makes no sense to permit the students to use whatever library function does absolute value. You're teaching bitwise operators, and the fact that they know of the library function doesn't help them learn bitwise operators.

I believe that because you're asking here, the reason for the prohibition isn't as obvious as in my example, but my guess (without knowing the specifics of the situation) is that the prohibited constructs are not what are being taught so, for that course, the students knowledge of a more advanced concept is irrelevant to the subject matter being taught.