There's actually another REALLY GOOD reason why they need to provide user-access to the hard disks:
What happens when your shiny new iMac breaks. Let's suppose it's completely died and you can get no life from it at all. Let's suppose it has to be returned to the factory for repair.
Are you totally comfortable with all of your private data going back to Apple? Maybe your machine will be stripped down and components might be replaced. Who knows where your hard disk might end up. Of course "best practise" would be for Apple to wipe the drive before repackaging it up and sending it out to someone else, but mistakes do happen.
I am not at all sure I can risk having personal information stored internally - unecrypted - on an iMac, bearing in mind I might one day have to send it away.
If the hard disk is user-accessible, you just pull it out and wipe it before sending it back.
Yeah this is a great point and never really considered.
I have a feeling that there are people within Apple who would want the iMac to work this way, but it was a Jobsian thing to have it all sealed. So there is a possibility it may be changed. If he were still around though, I wouldn't have any hope on that at all. We will see.