They only accept US-issued credit cards for use in the program. No one from the company will check to see if you have legal residence in the US, but they will check to make sure your registered card(s) is issued from a US bank.

This policy change came about 2-3 years back. I used to have both US and Canadian cards registered. When the change came, they sent me a notice that the Canadian cards would no longer be eligible for the program.