Well, you aren't technically allowed to share with your siblings anyways. Nor are you technically allowed to install on two computers.
If that's a deal breaker for you, then I guess that's that. Naturally they have to try to do something to stop outright piracy.
It does come with the added benefit (for the most part) of not coming with the "malware" that people hate such as Starforce.
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Not to mention lack of a need for DVD in drive.