Same here - I think the OP was probably meaning that if you log into the iTunes or App Store then you're prompted for your password as the app starts. Then, if you browse for an extended period and the purchase an app, it asks for your password again. As Gab suggested, if you purchase a few apps in quick succession, then you'll only need to enter the password once but it appears to 'time out' after a while - perhaps over concern that you've left your iPad unattended.
Tim