Because Apple needs to expedite their "record breaking" iOS 7 adaption headlines. So they've lowered themselves to using extortion tactics.
Not that most people aren't already on iOS 7, but it seems rather sleazy to force it on people (who don't want it) when it certainly doesn't seem necessary.
There could be another reason. My understanding is that they wipe the phone and run test software on it. They then need to put new software on it and since the system does not support falling back to previous versions of IOS they have to put the latest version.