I have a specific question about how US duty-free stores on the US/Canadian border enforce (if they do) the requirement for shoppers at those stores to take purchases out of the US.

When crossing from the Canada to the US, I've stopped at three Canadian duty-free stores, two in Quebec and one in BC (see below* for links). At all of these stores, the only normal vehicular exit from the Canadian duty-free store's parking lot leads directly to the US port of entry, so that anyone who drives into the Canadian duty-free store parking lot cannot then exit and return to Canada (via the normal lanes for passengers; I'm sure there are allowances for people who work at these stores). They have to proceed to the US border crossing.

However, on the US side duty-free stores don't seem to be set up this way -- the duty-free stores in Derby Line (VT), Sumas (WA) or Bonners Ferry (ID) (see below** for links) for instance, are located on public roads such that a driver could easily visit the store and then not proceed to Canada. They're all of course within a mile of the border, but the parking lot/roadways aren't set up such that you can only exit to Canada.

Given that, how do these US duty-free stores ensure that travelers who've purchased goods at those stores take those goods into Canada?

To make an air comparison, US duty-free stores in airports only give you your purchases when you're boarding on the jetbridge, when there's nowhere else for you to go except out of the US. Do you only get your purchases from US duty-free stores at land borders as you're driving across? That seems unlikely/infeasible.

*Canadian stores:
(1) (2) (3)
**US stores:
(1) (2) Sumas, WA
(3)