Because our government sold us out decades ago. Can't remember the name at the moment (Larson?)but there is a guy in Vancouver was selling pot seeds over the net and was given up to the US by our government because the seeds were being bought in the US.
It should be based on the physical location of the person committing the offence, not on the location where the offence is committed. If he was physically in Canada while selling the seeds online, then only Canadian law should apply to him.
I'd apply the same principle in reverse. If a person sitting at his computer in NYC robs a bank in Canada, I'd say he broke US law and not Canadian law. The Canadian authorities could cooperate with the US to provide them with all the information we have available to help the US authorities prosecute him under US law. But if he'd never set foot on Canadian soil, then it baffles my mind how he could break Canadian law without being physically present in Canada.
I'd apply the same to a person standing on the US side of the border with his rifle, pointing at someone on the Canadian side, and pulling the trigger to assassinate him. While it would make perfect sense for Canada to cooperate in the investigation, I don't see how he would have violated Canadian law. It would seem to make sense that he should be tried under the laws of the jurisdiction in which he was physically present at the time of the offence.