If the feds banned Americans likely the border would be shut down and Canada relies on it more than the Americans. At this moment supply lines cross it. The government is trying to preserve two things with the border being open for now:
1) supply lines that in many cases are critical for our response including medication
2) employment for those who can still work as the government needs the income to stop a financial disaster the will imperil what is a costly response.
A vindictive US president in this fragile time can severely damage the Canadians economy. Economic crisis damages emergency response- especially when it is likely to be a fairly long term emergency.
The government is balancing very difficult choices here.
Over simplifications should be avoided.
I think American tourists coming into Canada are being told they will have to quarantine themselves for 14 days if they want to stay in Canada. I hope this is true.