This doesn't mean that you don't know English if you speak to me in French, no more than it would mean that I don't know English if I communicate with you in French.
This is intended to encourage newcomers to learn French. I think it's perfectly legitimate. Quebec is the only province where French is the language of the majority and it's sort of the primary home of francophones in North America. So it is necessary to encourage the use of French and to make it the common language. This doesn't mean that the rights of the anglophone minority are not recognized; that's not the issue at all. If French isn't the common language in Quebec, however, it will not be the language that will enable newcomers to integrate and facilitate exchanges between all Quebeckers.