Here's a great reply to my similar question from Zyggy:
The trick is in the special status that Canadians have.. by Jay's treaty,Canadian citizens do not need a visa to enter the US for up to 6 months and vice versa. This is what makes it different that VWP. With VWP, the US has chosen to allow certain nationals into the US without the need to apply for a visa to be able to visit the US. In essence, they apply for their visa at entry (no pre-approval or screening). In this way, Canadian Citizens are very special. They have a right that no one else in the world has. CBP can revoke VWP for anyone since it is a privilege that CBP extends. They can't revoke entry without visa for Canadians since they have the right to enter without visa by treaty.
If a Canadian Citizen is entering the US for any other purpose than to visit, a visa is required.
When you get a K1 visa, you have put the US Government on notice of your intention. The question is of intent... once you get the K1, you have a visa that says "I 'm going to live in the US". That no longer makes you a visitor with strong ties to Canada.
For schitts n grins, I called both the Detroit Tunnel and the Ambassador Bridge. Tunnel guy says Doug HAS to activate the K1 on his next trip over to US, and ....you guessed it.....the Bridge guy said NOPE....just make sure you don't have anything with you and prove ties to Canada....neither one of these fine guards would give me a badge # of course