Considering the amount of people who still went ahead and traveled and went on cruises while the pandemic was underway, I would be worried too about people being irresponsible. The US still doesn’t have COVID under control in a way most people would find satisfactory. There’s people, wether through ignorance or selfishness, who would still travel to the US despite that. And I’m not sure I trust that everyone would isolate and monitor for symptoms when they got back, if the reports of the returning snowbirds were any indication.
Essential travel across the border was always maintained. And recently, they’ve broadened that to allow family members to reunite.
It’s also not like Canada is being unique in implementing border restrictions. Other countries that have a better handle over COVID like New Zealand, Australia, and Macau (off the top of my head) still have border restrictions in place.