The "residue" was up to a month old. It indicated that he had smoked marijuana at some time in the past month, and the lack of THC itself indicated that he was not currently under the influence.
This is not much different than the police arresting you (or me) for having a beer after we get home from work - after all, we were drinking and had been driving, right? Since the two no longer have to be simultaneous, why would the order matter?
So what? The voters through their representatives have decided to keep potheads and other illegal drug users off of the public streets while they still have dope residue in their systems.
Their roads, their rules.
More like arresting you for driving on the 27th of the month because of the beer you drank on the 1st.