So where does the money come from? Kelly Rummelhart of Gridley, Calif., has part of the answer. Her 4-year-old son was using her iPad to play the game and racked up $66.88 in charges on her credit card without knowing what he was doing.
Rummelhart had no idea that it was possible to buy things - buy them with real money - inside the game. In this case, her son bought one bushel and 11 buckets of "Smurfberries," tokens that speed up gameplay.
"Really, my biggest concern was them scratching the screen. Never in my wildest dreams did I think they would be charging things on it," the 36-year-old mother said.
She counts herself lucky that her son didn't start tapping on another purchase button, like the "wheelbarrow" of Smurfberries for $59.99.
Rummelhart joins a number of parents who have been horrified by purchases of Smurfberries and other virtual items in top App Store games. The 17 highest-rated comments on "The Smurfs' Village" in the App Store all complain about the high cost of the Smurfberries, and two commenters call it a "scam."
Apple introduced "in-app purchases" last year, letting developers use the iTunes billing system to sell items and add-ons in their games and applications.
This year, developers have started to use the system in earnest as the main revenue stream for many games. Of the 10 highest-grossing apps in the App Store, six are games that are free to download but allow in-app purchases. Four of those are easy, child-friendly games. Two of them, "Tap Zoo" and "Bakery Story," have buttons for in-app purchases of $100 in just two taps.
Capcom Entertainment Inc., the publisher of "The Smurfs' Village," says inadvertent purchases by children are "lamentable." When it realized what was happening, it added a warning about the option of in-app purchases to the game's description in the App Store, and it's updating the game to include warnings inside it as well. The game has retreated to being the fourth-highest-grossing app in the App Store.
The warnings may alert parents, but it's doubtful that they'd deter children who can't read and don't understand money. Also, the option to buy $59.99 worth of Smurfberries at a time remains. Capcom spokesman Michael Larson says "Smurfs" is no different from other games in this regard, and the bulk purchasing option is useful to adult "power players."
It's quite likely that most of the money pulled in by these games comes from addicted adults who want to quickly build their Smurf villages, bakeries, zoos and zombie farms. But there's a loophole in the in-app purchase process that children stick their fingers through.
Usually, the purchases require the owner of the device to enter his or her iTunes password. But there is no password challenge if the owner has entered the password in the last 15 minutes for any reason. That means that if a user enters the password for a purchase or a free app upgrade, then hands the phone or iPad over to a kid, the child will not be stopped by a password prompt.
Capcom and other game publishers have no control over the 15-minute password-free period, which is set by Apple.
Apple defends its system. Spokeswoman Trudy Muller says the password system is adequate and points out that parents can restrict in-app purchases. The parents contacted for the story received refunds from Apple after complaining, and praised the company's responsiveness.
However, there's reason to believe that the password timeout doesn't always work.
Andrew Butterworth of Brooklin, Ontario, was well aware of how in-app purchases work and of the password-free period. He was careful to let at least 15 minutes pass after a password entry before letting his 5-year-old son play with his iPod Touch. That didn't help, once he'd loaded "The Smurfs' Village."
"He came to me all proud and said he'd figured out a way to get all these Smurfberries," Butterworth says. "And as soon as I saw the Smurfberries, I knew that he'd purchased them using my credit card. I was amazed that he'd figured out a way to do it, because I was sure that he would have needed my password."
He had last entered his password on the iPod four or five hours earlier, he said. His son's shopping spree cost $140.
Chris Gropp, another Canadian, said he had not entered his iTunes password the same day his son bought $67 in Smurfberries, apparently without being asked for the password.
TeamLava LLC, the publisher of "Bakery Story" and "Farm Story," says the games follow all of Apple's rules and restrictions. In either game, it's possible to buy $99.99 worth of "gems" in one go.
The game publishers and Apple point out that device owners can turn off the option to conduct in-app purchases by going to the Settings app, then hitting the General button, then the Restrictions option. The parents contacted for the story had done so after being alerted to the purchases through their iTunes billing statements.
Butterworth was pleased with the refund, but still thinks "Smurfs" is a "scam."
"They make it a ridiculously difficult game to play, and you can skip all the difficult parts by spending money," he said. "I believe that they know exactly what's going on."