Well let's see here. How shall we characterize the competition:
IE: internal browser on the desktop OS that has 95% marketshare worldwide. Key part of how they drive users and webmasters to the rest of their products. Vendor headquarters: USA. Check.
Firefox: open-source but *surprise* - Mozilla has as much revenue as Opera does. ($70 Million last year) God forbid they actually draw attention to that. Their evangelism takes a different form. Vendor headquarters: USA. Check.
Safari: internal browser on the 2nd most popular desktop OS, by the company which sells 1/10 of all personal computer hardware in the world. Piggybacked on the software utility that runs the most popular music players in the world. No one's going to replace Safari any time soon. Vendor headquarters: USA. Check.
Chrome: another free browser by the new behemoth on the block, who has money coming out of their ears. They certainly don't need Chrome revenue, but it helps them take over the world anyway. Vendor headquarters: USA. Check.
Opera: the only browser offered by a company whose sole business is web-browsers, which actually relies on the revenue for same to stay afloat. Vendor headquarters: Norway. (Otherwise known as Europe) Check.
Now is it any surprise why Opera is the only obvious player here that would be A) inclined and B) have any credibility when it comes to making a case in a European court against Microsoft's Windows browser monopoly? They are literally the only company of all of them that really does rely on browser revenue to stay afloat. They are also the only European company represented here. (They are in the EEA, though not the EU)
So I say: DUH. Heckle Opera all you want, there are a lot of fairly obvious (to me) reasons why they *should* be the key petitioner in a European antitrust decision concerning Internet Explorer.