Its true that a DUI in the US is considered a misdemeanor but in Canada its considered to be a criminal offence.
Since the DUI is considered to be a criminal offence, the Canadian border agency can deny Americans from entering the country with a criminal record of any kind.
If you are an American and have a criminal record and would like to enter Canada without any issues at the Canadian border you will need to apply for a deemed rehabilitation application.
Pardons Canada helps Americans to obtain this application so that they are safe to travel to Canada.
Here is a recent article titled “DUI offence can bar you from Canada” which highlights the problems of trying to cross the Canadian border with a criminal offence: