I understand the logic with the change, and it's hard to disagree with the logic, but by the time the message makes it to the user's Outlook mailbox it has mostly been tagged and flagged as spam already (or not), and any additional processing by Outlook should, in fact, reflect the user's preferences of what is or isn't spam as the final consideration. Consider:
As a system administrator, I get a LOT of technical newsletters by email, and I've often had them misflagged as spam (SonicWALL's forum posts come to mind), so I've added the appropriate "sonicwall.com" domain to my safe list with the understanding that spoofing attempts from that domain COULD occur. I have to admit that since upgrading I've been wondering why my Outlook filters seemed inconsistent, and now I understand why.
Now I realize you're protecting John and Jane Consumer here, but, would it not have been simpler (and less disruptive to preference) to have added a dialog confirmation to Outlook confirming the choice to add a domain instead of specific address? Something like, "Adding a domain to the Safe Senders List can result in spoofed addressing reaching your inbox. Do you really want to add the domain to your Safe Senders List? Yes No"