The offers are popping up everywhere for cardholders: Win up to $1 million in weekly sweepstakes, just for paying with plastic! But consumer advocates have a warning: The get-rich-quick offers may only be enriching card companies, and encouraging consumers to change their behavior for the worse.

Facing the prospects of dwindling funds from tighter wallets and revenues lost to credit card reform, many major credit card companies have launched sweepstakes promotions. For Discover cardholders, each purchase counts as one entry for daily prizes and a chance at $1 million to be awarded at the end of the year. Citibank debit-card users have a chance at weekly drawings through Dec. 31 for prepaid cards valued at $10 to $10,000. These offers sound enticing, but when every purchase is a chance to win, the real danger is focusing on the prize and overspending, says Gerri Detweiler, a credit advisor for Credit.com.

The chance to win is a powerful incentive. In a 2009 study in the Journal of Marketing Research, consumers were given the choice of spending 50 cents on a bottle of soda or $1 on the same bottle that offered a sweepstakes with a $300 grand prize (odds of winning 1:600). Seventy-one percent opted to spend more and enter the sweepstakes. That pitch of found money is particularly alluring to consumers. We start thinking, If I go to the store and buy a TV, how cool would it be if Chase paid for it? says Noah Goldstein, a business school professor focused on organizational behavior at the University of California, Los Angeles.

For the banks, this is all part of the larger effort including offering cash for opening an account and touting 12-months interest free for transferring a balance on the part of card issuers to lure new customers and encourage current ones to use swipe more often. And contests are actually a cheap way to differentiate a card, balance transfer offer or other financial product because the costs are fixed upfront and running a contest is less expensive than, say, a big marketing campaign, says David Kramer, chief executive of The Marketing Agency, a firm that develops and administrates sweepstakes for third parties. Such offers waned during the financial crisis, but banks began rolling them out again in recent months, he says.

Want to win without spending any money? Per federal law, sweepstakes cannot require a purchase (or a significant obligation of time) as a condition of entry, Kramer says. For legal purposes, using your credit card or paying a balance transfer fee counts as a purchase, so companies have to provide another way to enter usually via postcard or a short online form. So before you sign up and hand over a card or transfer a balance in hopes of a win, look for the alternative to scoring those riches. Companies must disclose the alternative ways to enter in the sweepstakes fine print. Just have a magnifying glass handy.