Look at the identification. They are not identifying Opera as an actual virus, but as a type of suspicious program. Usually this is because of the background updater - to them, a program that makes changes to the computer without user interaction is dangerous, esprcially when you don't know what it is installing. As long as it is installing only appropriately-signed updates it is fine - well, if you can trust the signer. If some hacker managed to steal the signing keys, they could wreck your system.