First of all, it's not a labour monopoly. Second of all, certification, whether it's in Hamilton, or Toronto, or Kalamazoo, gives the union certain rights that flow from the Labour Relations Act. If one of those rights is the right to bargain with the employer, they can then make a deal. The deal governs their relationship. It is a matter of contract.
If you and I pooled our money and started Pierre and Bob's Balloon Company, and we went to the people who manufacture balloons and said, we will give you x amount of dollars in exchange for a right to be the only people who sell your balloons in Canada, it might give us a monopoly, but it is a monopoly we're entitled to get as a matter of contract, and should be protected by Canadian law.