All AT&T Android phones have that option (and checkbox) removed and non-market installs are not possible through the Android OS. Even the Dell Streak was blocked like this and it was 4 months before it ever saw an AT&T retail store.
Apps can only be installed via adb push commands, or through the official Android Market. (or through the Amazing side loader....blah, blah...)
Rooting your phone does NOT fix this.
So yes I do think so, and you are excused.
Originally Posted by mercuryzzz
excuse me but how is it possible they reach to block installation of third part apps? In all android phones you can go to setting>applications>manage applications: there is a check box that allow you to load apps from any source. Is it possible to block this feature? I don't think so.