Answering reader questions on the official Windows Partner Blog, Sony exec Xavier Lauwaert says security issues are to blame (the explanation comes in the comments section of the post):
Our engineers and QA people were very concerned that enabling VT would expose our systems to malicious code that could go very deep in the Operating System structure of the PC and completely disable the latter.
However, all is not lost, as Lauwaert says that Sony has listened to the complaints and will selectively allow XP virtualization mode to run on future systems -- although no specific details about models and timeframes are mentioned.
However, with the advent of XP Virtualization, there is impetus for us to relook at the situation and I can share with you that we will enable VT on select models.
While some have gone so far as to call for a class-action lawsuit on behalf of Vaio purchasers, this is an issue that's generally confined to the periphery of enthusiast PC users -- which is why the outrage has clearly taken Sony by surprise.