A King or Queen is nothing without its subjects. The Queen is only Queen of Canada because a majority of people either support monarchy, are indifferent to it, or don't agree with it but don't see direct necessity of changing the system.
Monarchy today is nothing like it used to be. 400 years ago, one would have been hanged on the spot or jailed for directly opposing monarchy. Today I can openly state my non-allegiance to Queen Elizabeth and nobody can do a damn thing about it. So long for the Queen's authority over me.
The Queen would only need to act once against the will of Canadians and the whole legacy of Canadian monarchy would implode upon itself.
On the other hand, the police, judges and all people working in the field of justice in Canada are Canadians who, for the most part, have the direct support of the Canadian population. We vote for people who have the responsibility of ensuring justice for all by administering the courts, police, army etc.
The democratic process in itself is what makes our laws legal. It's an equal power play between the population and those that are elected.
We should be thankful for this but also careful of the growing movement of apathy and cynicism when it comes to the democratic process.