I'm not clear why some of us feel that a private organization putting on an event that amounts to a freely entered contract between itself and participating cyclists should have to justify any of its rules and regulations to non-participants. Those are the terms of the offered contract set out ahead of time -- you are free to decline to participate if you don't like them. It's not like they took your money and then denied you entry when you showed up at the start line on a tandem.
If you want to play the disability card you could file a complaint with the British Columbia Human Rights Commission. But for all us other regular types, the old rule of "Don't try to crash a party you weren't invited to" seems to apply.
I'm assuming the "no tandems" rule was put there at the behest of the insurance company believing rightly or wrongly that tandems on the course increase the crash risk. If the Human Rights Commission orders the event organizers to remove the rule for future events (to avoid discriminating against those tandemists who might be disabled), then I predict that the insurance company will decline to insure it. The event will be cancelled and then everyone will be happy.