There's another reason variable-rate cards work in lenders' favor. Banks can set a "floor" in the fine print of a card agreement, meaning the customer's interest rate won't fall below a certain level regardless of how much the prime rate drops.

For instance, if a card's rate is based on the prime rate plus 5 percent with a floor of 10 percent, the rate on the card wouldn't drop below 10 percent even if the prime rate fell below 5 percent.

The current prime rate is around 3.25 percent.

Jacobson would not comment on Chase's policy on floors; Bank of America said it does not have floors on variable rate cards.

New credit card reforms may not be the only reason banks are migrating to variable rates, said Greg McBride, a senior analyst at Bankrate.com.

Card issuers typically start favoring variable-rate cards when the prime rate is at low levels, McBride said. This lets them benefit as the prime rate inevitably climbs back up.

But banks' preferences may not swing back toward fixed-rate cards even as the prime rate rises — at least not as much — given the new rules limiting lenders' ability to raise rates on a whim.