I have actually seen some exceptions to this.

Lennoxville, for example, is a town just south of Sherbrooke. Not sure if all, but certainly many, of their street signs are in English.

Being a newcomer to Canada, I'm am not entire verse in all the laws, but my understanding is that there is a Federal law that dictates all public signs must contain the french term. It does not stipulate English since the majority of Canada is already in English. In turn, the Quebec province made a law that stipulates all signs must be in French, along with a ton of other laws regarding the language such as an employee of a company in Quebec has the right to demand that his/her boss speak with them in French.

OK, so your next question should be "why"... This is where the things have gotten stirred up for me when I bring this type of issue up with my wife's family and friends. That said, the bottom line that I have observed is there is an incredible feeling of fear in Quebec that they will lose their heritage, culture and language unless they do anything and everything to preserve it. This is the primary driver for these laws. It's a tough problem which, in my opinion, generates a lot of contempt between alot of very very good people.

Hope this helps.