It’s not unfair. It’s business. They have no desire to increase their market share on the Wii, thus they don’t give the browser away for free. Plus, your Wii does not say it comes with Opera installed, thus you purchasing the Wii has no relevence here. As @StellarAirman says, do you expect ALL Wii games to be free because you paid for the Wii? Of course not! The browser is the exact same, except you don’t have to go down to the store to buy it.
It takes more effort, which means money, to port their software to the Wii. This is made up for by people paying for the browser. If you don’t want to use it, you don’t have to pay for it, but if you want to use it on your Wii, then it is only fair to pay for it properly.