Items that have faults not caused by the user and are within the return policy do not have a restocking fee. If they are charging you call 1-888-BestBuy or its Canadian equivalent, I can't say what they are doing is wrong because I don't know Canadian laws enough to say what they are doing is absolutley wrong and I don't know your situation. If they are charging you the restocking fee, you fucked up somehow.
Oh and using the excuse "IT'S TO SLOW" for a computer is unacceptable, the salesperson probably tried to tell you it would be slow and you chose to ignore them.