Android Wallet and Sweeping

[Kiwarian]

The Android wallet says.

You are about to sweep a paper wallet or coupon. This will move all coins from that paper to your wallet on this device. When the transaction is confirmed, the paper will be worthless and should not be re-used for safety reasons.

Why can a paper wallet only be used once? If I add more coins to the wallet I should be able to sweep it again later? Am I right?

 

[Ftoole]

You can but you risk that the software you sweep the paper wallet with could compromise the private key.
I have tons of paper wallets they were all made on a machine that never touches the internet the thought is keep the private keys offline then they are safe.