One of the drivers for me purchasing WW Wish Cards is to get the Amex Rewards 3x points bonus for "supermarket spend", which certainly works when bounced to the WW site for purchase. I expect the Amex bonus points would be at risk when purchasing from other sources???
Naturally, I understand that not everyone is buying them for the same reason, so won't be a consideration to many people. For me, if no Amex bonus and a reduced discount down to 3%,, I would need to reconsider the overall benefit to pre-paying for my groceries and vehicle/personal fuel needs.