I'll explain what happened here so everyone understands the situation. Denver snapped the ball and Jake Plummer threw a pass down the field and the Pittsburgh defender hit the receiver before he caught the pass. The official correctly threw the flag for pass interference against the Pittsburgh defender. Someone else on the field for Pittsburgh objected to the call and argued the call, which doesn't warrant a penalty flag. What did however, was that this same Pittsburgh player took his helmet off and threw it on the field, which was an automatice penalty for unsportsmanlike conduct, and would have been a 15 yard penalty. Since the pass interference penalty was already called and was the greater of the two penalties in yardage (about 25 yards), the Broncos had the option of which penalty to accept, and quite logically took the pass interference penalty. Had the unsportsmanlike conduct penalty been a late hit on the quarterback after the play was over, if a punch was thrown, or was a cheap shot or late hit out of bounds, then it would have been added to the pass interference penalty.
On numerous occasions there has been more than one penalty called on one team, such as offsides on the defense when the ball is snapped, and then a face mask penalty on the defense on the tackle. In a case such as that, the offense will get their choice of which penalty to accept, since an inadvertant face mask penalty is 5 yards, and an intentional face mask penalty is 15 yards, each of which carries an automatice first down. So why accept the 5 yard offside penalty, when either face mask penalty carries the benefit of an automatice first down in addition to the yardage.