Because while Microsoft has every right to disable their software, they have NO LEGAL RIGHT to turn my computer on or off. That is *MY* hardware. You turn it off without my permission and I will have you in court for unlawful trespass, unauthorized computer access, unauthorized use of property, and a whole slew of other Federal charges.
You'll be laughed out by the first judge to hear your case, too.
If MS has decided to disable Windows 98 because they no longer support it, then MS would likely have been in breach of contract and paying out the nose. However, the RCs were distributed with clear notice that the software would only function for so many days and not intended for production use. You gain no superior right to Windows by installing it on your home hardware.
In short, MS has every right to shutdown the RCs. If you don't like it, ask any /. reader to help you install a Linux distro.