Why are Canadian coins allowed in the USA?

By GovMint :

While not legal tender in the United States, Canadian coins find a degree of acceptance, generally by error during transactions, particularly in areas close to the Canadian border or in locales frequented by Canadian tourists. This informal acceptance is primarily due to the practicality and convenience for consumers and businesses in these regions. The similarity in size and appearance between certain Canadian and U.S. coins, such as quarters and dimes, contributes to this phenomenon.

However, it's important to note that this acceptance is neither universal nor official. The U.S. does not recognize Canadian coins as legal tender, meaning there is no obligation for any business or individual to accept them.

Sources Used:

Will the U.S. Accept Canadian Coins? (What You Should Know) (thefinancialgeek.com)