If Walmart really cared and wanted Samsung pay still, they would have it. They aren’t your run of the mill retail client and thwir POS provider is not going to be allowed to go in and add or take away payment options on their own particular whim or preference.
I can’t understate how trivial it is to compromise a public system with Mack-truck-sized vulnerabilities. Payment systems are NOT immune. I’d rather they err on the side of caution.