American Express Accepted Everywhere

Over the years I grew up with American Express being positioned as a prestigious, high status credit card brand.  They worked so hard on it that they made sure their cards were accepted only at places of high status - luxury restaurants, hotels, etc.

Then I saw this at Eaton's Centre in Toronto the other day.  Made me think: "Burger and American Express?  Really?" 

An Amex black card may even have been a goal for some aspiring businesspeople out there, but now that they can use the Amex card at the local burger joint, what does this do to the brand?

I wonder why Amex made the decision to go mainstream and compete for the same customers that are currently catered to by Visa and MasterCard.  They had a nice little niche - is it because the high status, high spending group is shrinking?  Or they feel like they've saturated that market?

What do you think is the reason for this switch in brand positioning?