I think there are two parts to the answer.
The first one, as Professor Dissou mentioned, is that essentially, the broader the pool of potential emissions you're considering with a measure, the lower the cost of addressing the measure. Essentially, if you think of it this way—let's not talk about distributional issues but just about the cost of action--in identifying where the cheapest emissions reductions would be, then the bigger the area you're looking over, the more you'll find cheaper emissions. Fundamentally, that's the attraction. From a Canadian perspective, province-by-province legislation or an approach that's province by province or territory by territory doesn't make a lot of sense. It's the same type of argument with respect to going over international boundaries. If there are cheaper emissions reductions somewhere that you can get credit for, or at least partial credit for, so much the better for the effort.
The difference, however, is in the transfer of wealth. As you buy a credit from somewhere else, you're essentially transferring some of your purchasing power to that other jurisdiction, if you want, or to people in that other jurisdiction. So it creates a trade-off.