Your statement is contradictory. Since it is MS's site, they can do whatever the hell they want. There is no right to view a specific site with a particular browser.
Don't let any hatred you may have of MS cloud your judgment here. There's nobody's data being screwed up; there's no harm being done. MS simply wrote a site that won't display using a browser they don't like. That's the market. Opera users can go to other sites, and if those don't work, they need to get another browser.
If you cant see this was unethical then I am sorry for you and perhaps I am not the one letting my 'loyalty' to a certain set of software cloud my judgment.
Using underhanded tactics or lies to disparage another's product in the marketplace is subject to a civil suit.
By purposely breaking Opera on a site visited by millions, Microsoft tried to make those users with Opera think their browser was broken, casting doubt on the functionality and usability of that browser, which can be incentive for people to switch to a more "compatible" browser, such as IE. Such action is bad enough, but coming from a convicted monopolist known for anticompetitive behavior is that much worse.