The goal is to learn, and the point of the exercises is to teach a specific concept. If a student finds a different way around the problem, that may show that they're already proficient in other skills, but they haven't necessarily learned the concept being taught in this class yet. A good instructor would probably acknowledge the solution, but add extra boundaries to the task to get the student to explore the problem in a way that lets them encounter the testing difficulties discussed here.
It's like smuggling a calculator into a class about mental maths strategies: you'll probably do very well in the final test, but you won't have learned anything!