Citi issues the HHonors VISA and must share the processing fees with VISA. American Express operates its own processing network -and- charges higher transactions fees than VISA.
Ok, that makes sense. It's too bad though, as a HH Visa (at 3 pts/$) would have been a great 2nd card to carry along with my SPG Amex. All too often I get rejected (by the vendor) when I try to place an order on my Amex...and as of now I go to my Citi AA MC. It kills me when I lose out on 50k or 100k Starpoints in one shot. So the HH Amex won't work as a backup, but rather another option to diversify my spending with. Nothing wrong with that.