Because as far as I know (I could be wrong), Oakland did not have their President announce to the press they may release Brantley within an hour of drafting him...investigation pending or not.
The point is, Cleveland clearly had doubts to have made that statement so quick (to cover their butt)...and that makes it look like a stupid move and a waste of a pick (value pick or otherwise).
Also factoring into the situation (although not the comedy value) is the fact Brantley has been charged and forced to file his plea (not guilty) this week. Clearly, that tells us the cops and DA feel there is enough evidence to proceed to trial and/or plea bargaining in Brantley's case. No such situation existed at the time of the pick of Conley (or at all...at least yet in his case).
So, the Browns pick was more risky, they had to treat it that way by immediately saying "we may release him" immediately after making the pick and that makes them look both inept and funny.
Side note: They could have told the press "Listen, we don't know the facts of his case, he assures us he is innocent and he's passed a polygraph for us. So we decided to roll the dice on a value pick using a 6th round pick to get what we felt was 2nd round talent. If he is found guilty or our investigation finds anything we don't like we'll drop him and be out the #1 6th round pick. It was worth the shot." They did not. They went the other route making themselves fair game for laughter.