Even if the app is free, it may show as "Purchased" after you download it. You shouldn't use the "purchased" label as an indicator that you have been charged for anything. To know whether you've been charged, you need to look at your email receipt, your purchase history, or your credit card statement.
If you look at your purchase history, you'll see that the free apps are listed as "purchased" for $0.00. To see your purchase history, use iTunes on a Mac or PC, to go to the itunes store and click your account in the upper-left hand corner ( it may be in the upper-right corner on some versions of iTunes). If you find you actually have been charged for something that you didn't intend to buy, you can click the "report a problem" button to contact Apple.