Ethereum itself is not an ICO. ICO is something other projects can do on Ethereum (or on other platforms). There is actually some big differences and the rules are different. Bottomline: you are free to create your own cryptocurrency and pre-mine any large amount of it to yourself. Good luck making such a good one that other people will consider worth using and buying it, knowing about your premine. (And I even hope you succeed, because it will mean that you have created tremendous value for the world. But the chances are you won't.)