It's fairly trivial (conceptually) to close the border to non-commercial traffic while allowing commercial traffic through, given that both are separated physically at the border and you need paperwork to go through the commercial lane anyway.
> Unless they think they have significantly fewer cases than the United States, it doesn't make sense to block travelers.
Absolutely, given that Washington has mismanaged the crisis, and in the US there is often a monetary cost for even seeing a primary care physician, plus no mandated sick or vacation days, and the numbers so far reflect this.
The biggest concern, in my opinion, would be people who try to come to Canada in the hopes that they can get sick here and recover somewhere that isn't going to bankrupt them as a result. Not that I want our southern neighbours to die in the streets while we lounge in relative medical luxury, but risking overloading both systems would be catastrophic for everyone.