But requiring foreign visitors to have certain vaccines is much less politically fraught. If you don't like it, tough, you don't get to enter said country.
Asking what is the science behind these decisions is being disingenuous and you know that. Please stop.
As to scientific or not science, the science is in. Vaccines reduce the spread and severity of COVID and governments generally attempt to mitigate the risk of dangerous things happening to their people needlessly. Why are mandates constantly changing? Look to the knowledge available to the decision makers when they set policy. Most governments try to thread the needle between positive economic and health outcomes, but to be sure, nobody know the right answer. All decisions have consequences.
Obviously an unvaccinated trucker from Canada cannot enter the US and return until both the Canadian and US restrictions are lifted. It doesn’t matter in which order they are lifted, but both need to be. Since they are Canadian citizens obviously they are protesting the Canadian component of the travel restriction.
On a side note: How does it make sense that an unvaccinated trucker in Canada can legally deliver goods in Canada, and an unvaccinated trucker in the US can legally deliver goods in the USA, but they cannot deliver goods between the US and Canada. What is the science behind this exactly?