Curious to know if it's even possible to know whether a driver has manually sent a text message / composed it via voice / given it to the passenger to compose text.
At least with breathalyzing, it's binary. You're either drunk or you're not, there's no middle ground or room for interpretation.
Breathalyzing is binary but "being too drunk to drive safely" isn't, they just made testing binary for the law's sake. It's possible to be under the limit for a test yet still have enough alcohol in you to slightly impair your driving if you're not used to drinking, it's also possible to be able to drink quite a bit over the limit and still be sober enough to drive safely, but the law just looks at averages and picks a spot to make it binary.
From the article: "Further analysis, which might require a warrant, could be necessary to determine whether such usage was via hands-free dashboard technology and to confirm the original finding."
As the other commenter noted, "to drunk to drive" has way more room for interpretation than "sent a text before an accident", yet the law seems to have arrived at a reasonable compromise.
Note that sending a text message via voice and is still distracted driving, and some studies show that it's just about as dangerous. It's currently legal, but some jurisdictions are contemplating banning that too.
At least with breathalyzing, it's binary. You're either drunk or you're not, there's no middle ground or room for interpretation.