I wonder what would have happened if you had tripped and sprained an ankle?
What's a runner expected to do when the entire point of the activity is to travel light... very light, push the pace, build endurance and challenge the body? Do you carry a first aid kit? Pack a shelter in case he/she injures themself and can't make it out? Bring insulation for the night in the event you become stranded, injured, or lost? If you go down this route too far you venture right back into backpacking territory and it becomes something other than trailrunning.
Granted... a SPOT device, headlamp, and essentials only First Aid kit tucked away in a hydration pack might have been a safe(r) measure for a self-propelled alpine outing, but I think that quickly starts to encroach into the territory of backpacking and chips away at the core of what trailrunning is all about. Is one to pack a certain amount of gear when in the mountains regardless of the activity?
I think you nailed it Craig, the more you take on your walk, the slower you travel, and the longer you are exposed to the elements. Year round, there are endurance athletes and conditioned enthusiasts alike who run the Grand Canyon Rim to Rim to Rim in less than 24hrs., the fastest doing it in a smokin' 8hrs! Then you have others who take significantly longer (by choice and often physical limitation) to do so and due to the extended nature of their trip have to rely on overnight and multi-day permitting, food, stoves, shelters, insulation, and other "just in case" items and necessities to get them through their outing. These are tools necessary for the activity, this is backpacking.
Trailrunning is a sport, backpacking in most cases is... dare I say (gasp), not a sport and this is the big difference between the two.