The App Store app makes it easy to buy programs for your iOS device without relying on iTunes and syncing. But there's a gotcha. Once you enter your iTunes Store password to make a purchase or even download a free app, the password is cached for several minutes.
If you accidentally tap to purchase another program within that period, you're not asked again for your password. Apple does prevent purchases that could be caused by a nervous tap: you have to tap a price, pause a second, and tap again to buy; if you double-tap quickly, the purchase is canceled.
Where this really becomes a problem, however, is with in-app purchases, as Adam explained in "Be Aware of iTunes Password Caching," 14 July 2010. Log in to buy your child a program, and then hand your iPod touch or other device to him or her to play it. Whoops! Many apps feature in-app purchases for upgrades, like additional drawing pages, new game levels, and advanced features.
Adam noted several suggestions about how Apple could improve on the situation, but I recently found one practical solution; TidBITS readers suggested two others in the comments on this article.
For a blanket ban, one reader explained, launch the Settings app, tap General, and then tap Restrictions. Enter a 4-digit numeric code required to restrict access to various features on the iOS device (and re-enter to confirm it), and then you can disable all kinds of things, including in-app purchases and installing apps.
This is an all-or-nothing solution. When in-app purchases are disabled, the only way to make a desired purchase is to return to Settings > General > Restrictions, enter the code, and re-enable the ability to make such upgrade payments. Otherwise, you're just told you can't.
Another way to prevent paying for items within an app or buying apps is to log out of your iTunes Store account immediately after use. On the App Store program's Featured, Categories, or Top 25 tabs, swipe your way down to the bottom. There's a Your Account button that you can tap, and then tap Sign Out. (That's also the part of the screen where you redeem iTunes credit or promo codes.)
Alternatively, you can instead launch Settings and tap the Store item in the third group of preferences; tap Sign Out in the screen that appears.
Apple needs to add more granular purchase controls and a more flexible lock against this problem, to judge from message boards and reviews of apps that offer in-app purchases. It's part of the larger issue that Apple assumes only one set of permissions is necessary for a given device, because only one person uses it most of the time. That's clearly not the case.