It is not exactly mandatory, but the company nags the employees who haven't passed the exam yet. It is also good for your performance rating, because the managers are advised to consider the result of the exam when doing performance review.
For what it's worth, their recruitment tests for fresh graduates in India is similar in that it doesn't allow usage of any STL. At least, that was the case 3 years or so ago.
It must be the same test. There are three levels in the test and new employees are given the first level.
Just out of curiosity, did you take the exam? Was the process satisfactory? Did you have a feeling that the test correctly represented participants' skills?