Because you can't trust downloaded versions of Windows, and it's not worth paying full price for it.
Simple really. Why pay more?*
*rhetorical question, doesn't need answering. I'm aware of the legal reasons but the simple fact is, if you can buy the software for a fraction of the price, and microsoft take it upon themselves to reactivate it (they must be aware of the lies) then the consumer has more of their hard earned cash in their pocket.
It's not my job to audit Microsoft's activation process, and i'm not going to feel guilty for taking advantage of a situation that allows me to save money. We're ripped off from every angle every day as consumers because we don't get a choice in the prices we pay. Well, now we do.
If MS have a problem with it, don't reactivate products.