Vince McMahon is loyal to his athletes. However, when an athlete tries to discredit his federation by discussing steroid abuse etc, McMahon will strike them down with unrelenting fury.
Yes, Cena is a smart steroid user. To this day he has not let-on about his abuse of drugs. Hence, Vince is still loyal to him.
The message is, you cross Vince and your wrestling carrer is in the sewer!