Not to defend the legality of the tackle, but in a field position game it makes a lot of sense. It stones the runner's foreward momentum and drops them backwards. This as opposed to wrapping the ball carrier's legs up from behind where they fall foreward and gain another two yards. That's often the difference between the punt team hitting the field or four fresh downs to work with. Horsecollar tackles are legal at the collegiate level because the level of athleticism is much different. In the pros, where guys are carrying 220 (frequently more) at a 4.4 clip and the body is working to it's physical limitations, any tweak to a runner's gait can cause massive injury. That's the reason the rule exists at the professional level. It's not in the nature of the hit to maliciously attack the ball carrier, just to stop them from making any further progress.