Each copy of the software needs to be initialized with a token. Google tries to limit you to have one copy initialized at a time, but I'm not too sure how effective they are.
Is this an iDevice limitation? The android version doesn't connect to Google's servers at all, so there would be no way for them to know you've setup multiple copies.
Generally it is because sites only show you the seed once; you can't get them to give you the seed again. You can just write down the seed or enter it into multiple devices if you know this when you set it up initially, though.