While it is true that the vaccine is not a 100% guarantee, the virus does not transmit in a vacuum. You spread it via droplets (coughing, etc...) with a high viral load. No symptoms, low viral load = low chance of spread. The vaccine severely reduces the ability of the vaccine to multiply inside of you. If your viral load is low, and you have no symptoms, it's almost impossible to infect another.
The Moderna and Pfizer vaccine work by targeting a specific part of the virus which has not changed in the new variants. Early data shows that the vaccine is effective against them, though of course it can only be conclusive by a phase 3 clinical trial, and that's almost impossible to do under controlled conditions right now.
Asymptomatic infections for the vaccine are very very low - 0.1% for the moderna vaccine as per their clinical trial.
Yes, there could be new mutations we don't know about. Yes, it's still possible to be infected while on the vaccine (though 1/20 as likely). Yes, there is never any 100% guarantee. But the vaccine severely reduces the odds. It's not logical to completely ignore that in crafting public policy, not anymore than it is logical to ignore other transmission risk factors.
The only reason Canada has so far, is because the vaccination rates are so low, that there is no reason to come up with rules treating vaccinated vs non-vaccinated differently. But as the vaccination rate increases, that will change.