I dunno, I think maybe it's even simpler than that. Should the Army brass have broadcast details of a ~rumour~ that it was friendly fire before an investigation could flush it out? Of course not. As I recall, the initial reports were pretty sparse. Tillman was killed in Afghanistan in heavy fighting as his unit was attempting to take some hill or other. ...and not much beyond that.
Tillman, for good or ill, was a higher profile soldier than most due to his fame. So it quickly became a national story. I just think it is really unfair of the media to allege that the Army did some kind of coverup or lied about the circumstances. I don't see it that way. It was a lousy situation and it took time to sort it out. Nothing new about that.
Trust me, I'm no fan of the media, and I think Tillman attained American Hero status the day he exited the locker room and walked down to the recruiter's office. For that matter, the vast majority of our service members are heroes, but Tillman's stature made him more visible than most...which is precisely why they should have been far more circumspect about the manner in which his death was handled.
The Army wasted little time awarding him a Silver Star...something that should not have been done without a thorough validation of facts and circumstances.