You do realize that many states have such "distracted driving" laws, correct?
And yes, it is a subjective manner, just like a lot of the enforcement of the law.
If you are swerving around the road and the cop notices, it's pretty easy to see that you are driving distracted/recklessly. Who cares what the reason is? It could be because you are eating a cheeseburger, putting on make-up, talking on a phone, or getting road head from your passenger. If you can't drive without being a danger to those around you, then you need to be ticketed for it.