Because it costs MS nothing to print two more license keys and is $40 more then they'd get if the home user just installed the one copy on all their systems.
If that is just an upgrade version, I think I would buy one OEM version for my desktop, and then this for 2 laptops in the house and maybe the netbook.
Well, it's again a US only thing... Upgrade for cheaper ? The world best economy needs it ! Central Europe doesn't need such thing, they can buy the box for full price, even if that means 1/4 of average wage (199€ for W7HP vs 750€ average wage). Family pack ? The world best economy needs it ! Other just pay the standard price...
Ah crap, why do i even bother, it was allways the same, even before we were in EU, so don't come up with EU crap.
Apple had family packs for some time, so why not cash in on it from SM as well..
smart move now considering how many people have more then one system in their house, MS really wants to get rid of XP and get Windows 7 out in mass numbers and this is a great way to do it!