To emulate JScript in such detail the authors have stated that they need certain browser functionality. They use a Mozilla extension to the ECMAScript standard, the ability to define "getters" and "setters" for object properties. Opera does not support this functionality (mainly because it is non-standard – not in the ECMA specifications – and not widely used.)
On the other hand, Opera has worked hard on being compatible with many of IE's extensions.
The irony! Microsoft's script library basically tells Opera "sorry, but you've spent all those years implementing the wrong standards violations! If you had copied Mozilla's violations rather than IE's we might make live.com and start.com work for you"..