Thats the point - there are literally 100s of other ways to be just as (if not more) impaired while driving and be completely legal. We don't parade those issues around (namely because the big pharma companies would like people to not realize just how impairing their products are)
The difference is that there is a large enough group opposed to legalization that they make this issue a primary one in their fight against legalization and blow it vastly out of proportion to the actual scale of the problem.
You can actually be arrested for an 'OUI' in some states, an Operating Under the Influence, for when you are clearly impaired, but fall below the legal cutoff for a DUI. This would likely necessitate being the same sort of thing, a legal recognition that you were driving recklessly, and you exhibit the signs of recent marijuana usage (or if we have some way of actually measuring it, then that too).