Technically they cannot. They fall under the Directorate of controlled goods in Canada, and likely fall under the US export regs as well. That is not to say that guys haven't brought them in, just that they are subject to seizure. Places like milarm had them for sale with the rifles a decade ago, and they still surface from time to time here.
When the 4" smoke discharger barrels were available a decade ago out of the US, a number of the collectors I know had them seized by customs. It was a 4" piece of tubing with one end capped and a threaded hole. Most made it in, some did not. And this was in the pre 9-11 days.