If you are talking about Bell TV and Star Choice inserting commercials into US broadcasts, then you need to blame the US broadcasters. The same thing happens in smaller markets in the US - the broadcaster has a "national" commercial that the "local" re-broadcaster is allowed to replace. In this case, the re-broadcaster is Bell TV and Star Choice.
If you are talking about Bell TV and Star Choice replacing the US broadcast with the Canadian broadcast of the same show, you can blame licensing agreements. For example, CTV pays for the rights to show CSI in Canada. If you try to watch it on CBS instead, you can't because the CTV feed gets put in place of the CBS feed during that show. (The CRTC is only involved in that there is a long list of rules defining when and how they can make this switch.) The re-broadcaster (Bell TV, Star Choice) is forced to make this switch - they don't have any say in the matter.
You can already see similar agreements in place online. Try heading on over to comedycentral.com from anywhere in Canada and you will be redirected to thecomedynetwork.ca. The CRTC and your ISP didn't have anything to do with it.