You may be right that some women are forced to wear the niqab by their male relatives.
1.  What about these women who say they are not?  http://nationalpost.com/news/canada/canadian-survey-of-women-who-wear-the-niqab-reveals-choice-may-be-a-bit-of-a-youth-movement
2. If they are already being forced to wear it, what is this law meant to accomplish exactly and what do you think the likely effects will be?  Allow me to theorize that their overbearing male relatives aren't going to say "oh geez, I guess I better let you take it off and go about your business".