I have once asked google about it. They said that google do contextual advertisement. This means that google targets the content of your site and displays relevant ads. You can't place ads of some other company which also do the same function as of google. I am not sure about CJ but you can check it yourself. Like some CPM paying companies like cpxinteractive and value click don't hit your content and just displays defined banners. so they can be placed with google. this is what google told me. I am not the owner of google
Actually, theya re even allowing other contextuals to appear as long as those ads don't appear the same. You must send them to be sure which is allowed and which isn't though.