I worded my original question poorly. I'm not disputing the call. It was ticky tacky, but they were more than 1 yard down field. I'm not even whining about it, it didn't decide the game. I'm really asking why it's a rule? What purpose does it serve? Holding make sense. I've never played linebacker or safety. I'm really just looking for an explanation of why it's in the rule book?
It is my understanding that the rule is in place to prevent O-Linemen from posing as receivers. When a team lines up pre-snap, there needs to be 7 players on the line of scrimmage and only the 2 outside players are eligible. If any of the inside 5 player go downfield, this could confuse the defense as to which players need to be covered which could leave an eligible receiver uncovered. This become even more crucial in todays football where a TE or RB blocks before releasing late.
In college football, a lineman is allowed to go downfield up to 3 yards past the line of scrimmage. In the NFL, it is only 1 yard.
Another reason why this rule is in place is to prevent a team from confusing the defense as to whether they are running a pass or a run play. On run plays, linemen are allowed to go downfield. A play is not considered a pass play until the QB's arm is going forward. Therefore, a lineman is allowed to be downfield until the QB attempts to throw a pass. If the pass is behind the QB, then it is a lateral and is still considered to be a run play and the linemen can be downfield prior to the lateral.