Why official transcripts should be directly posted to the employer?
There are a number of security concerns:  


The postmark will match that of the university 
Many universities' official transcripts contain authentication features
that can't be easily replicated -- deterring fakes.
The receiver can contact the university to verify that the transcript
was indeed sent on the date listed on the transcript and that the details inside are correct.
The transcript may contain a letter from the registrar with a signature.
As in responses 2 &amp; 3 this can be used to make authentication possible while
deterring duplication.


The question I'm left with is why do you ask?