The left is angry at banks for not managing their credit risks well enough, loaning money to people who couldn't pay it back. The implication is often that this was all some sort of scheme to get working stiffs into debt slavery. Now it looks like American Express may be cracking down on credit risks, and Kevin Drum is mad . . ."Mad" is used here, I believe, in the same sense as "Mad Hatter" or "Mad Scientist" . . . or "Mad Magazine."
Thanks to Smitty for the mad tip.