Because of access levels. You may install an extension that requires no access to anything interesting (hey it's just harmless css), but may refuse to install s

Because of access levels. You may install an extension that requires no access to anything interesting (hey it's just harmless css), but may refuse to install something that wants to read your dom. Another fun thing: user css / theme extensions where you only install themes. Themes are not dangerous, are they? It's just eyecandy.