w.r.t. If he was really innocent, why wasn't he reinstated with full back pay and seniority ?
Probably because he has brought the company name into 'disrepute' - regardless of whether or not he was under the influence of anything.
More often than not if you do anything which plasters your employers name across the 'news' you will very likely get the boot, either that or be invited to 'fall on your sword' - albeit that the end result is the same, reputation in tatters, and a strong likelihood of no job thereafter.