Your subject line, "American issue, not Walmart specific", is completely wrong.
I've been able to use Apple Pay / NFC payments here in America for a long time at a lot of stores - my local hardware store, my pharmacy, both of my local grocery stores. But some specific stores - Walmart and Home Depot come to mind - made a deliberate choice (in the U.S. at least) to try and push their own payment "app" rather than adopt the standard NFC payment system. The main driver for this appears to have been they did not like how they couldn't collect as much monetisable personal data from customers using Apple Pay (or Google or Samsung).
The fact that, with Walmart at least, this appears to be an America-only issue makes me wonder if it's due to the different regulatory environment in Canada - perhaps they legally couldn't do it there.