By default, once you enter your Apple ID for a purchase you have 15 minutes to download as much content as you want without being prompted for your password again. Apparently the same things holds true for In-App purchases. I downloaded a free app from the App Store and then went to add some bux to support my Pocket Planes obsession. Sure enough, I was able to spend $.99 without entering my password a second time. The App Store assumes it's still you.
So how do you stop this from occurring? Easy. You need to enable your restrictions through the Settings App. In "General", choose "Restrictions". Select "Enable Restrictions" on the top, then enter a four-digit password. Make sure you remember the number or write it down somewhere: Apple can't reset it for you. Then there are two things you can do. Both are under the "Allowed Content" section.
- Turn off the "In-App Purchases". This would keep anyone, including you, from purchasing add-ons. Which is good if you are obsessed with Pocket Planes.
- Change the iTunes password requirement to "Immediately". This setting means that you can no longer enter your password one time and then download addition apps without it. Every time you download, even consecutively, it will ask for your password.