Where is this rule. I just looked at the Highpower rulebook as it is published on the NRA's website, updated January 2016, and don't find any mention of chronographs or radars. This does sound like something the NRA would do though.
As has been stated, I don't see any negatives to having a device like the Labradar or Magnetospeed on the firing line. Having used both, and the Labradar in a match, I don't see how they could block another shooter's view of anything. I don't see how they would slow down a match, if you don't get it set up during prep, you take your time away from shooting to mess with it. Since I have seen high velocity shots go low and low velocity shots go high, I don't see how knowing the velocity of the last shot fired helps with the next shot. All these devices do is help a shooter keep better trending data of their equipment to, possibly, allow them to evaluate their equipment and reloading techniques for future matches. Should we outlaw spotting scopes since they clutter the line, and you already have a scope on your rifle in F-class?
Saying that we aren't allowing them because it runs the cost of the sport up is crazy when we have $2,000 to $3,000 scopes, $1,500 rests, $1,500 - $2,000 actions, $300 triggers, $1,000 - $2,000 stocks, etc. How can a $500 chronograph be the thing that puts us over the edge.