Though Bell may buy the rights to telecast the game, isn't it your local cable company who picks off the signal from the ether to deliver it to your TV? As such, Bell mandates that provider do the simsub to prevent US commercials being seen in your home.

So it is Bell, or other rights purchaser who takes responsibility? Without the cable capture there's no issue. Over-the-air capture from your antenna is not prevented. Is this not signal piracy by the cable companies who are kowtowing by simsub with the endorsement of the CRTC to poach the commercial time?

Remember it was not long ago that Global (?) took the extra step by ghosting commercial billboards onto US sports events during the live telecast, making it appear that Canadian advertisers bought space in US stadiums. Regulators put an end to that.

How is simsub different from this, and why does the CRTC allow rights owners to poach the commercials at all?

No stopping viewers who WANT Canadian commercials -- just watch CTV!