The practice of turning tourists away for this is rooted in the scary period after September 11, 2001, when government agencies around the globe began tightening border restrictions. At that time Immigration Canada officials began to use our National Crime Information Center with an eye toward spotting terrorists and other kinds of troublemakers. It's not known how many terrorists have been stopped as a result, but a substantial number of tourists with blemished backgrounds have been. The attempt to keep out terrorists has turned into a system of preventing people with what are called indictable offenses from entering the country. Indictable offenses roughly translate into what are called felonies under US law.
The complication with this is that Immigration Canada uses Canadian definitions as to what are indictable offenses, not US definitions. This is what is ambushing many American tourists. Driving under the influence, for example, is an indictable offense, or felony, in Canada, and so are some charges relating to Domestic Violence. Those crimes in the US may or may not be felonies, depending on the circumstances and on state law.........(continued)