Do people realize not realize that DUI includes things besides alcohol? Blowing a 0.0 doesn't really matter in the least.
I think people generally have a higher social tolerance for medication that's prescribed for medical reasons, and either know someone or have suffered with addiction themselves, and know that you don't make good judgments under those circumstances. Not that it doesn't make it a DUI or an OUI or that he doesn't deserve the punishment of a DUI, or what have you, but the idea of someone getting hammered at the bar and then driving home intoxicated is generally thought of as being worse than someone taking medication that they're supposed to be taking for some medical reason, and operating a vehicle. Both are DUIs, but there's a greater social stigma for one than the other.
It's similar with other forms of drug use or distracted or unsafe driving. Getting into an accident because you were distracted because you're texting and getting into an accident because you were distracted by a screaming toddler in the back seat are both distracted driving, both are unsafe, and result in accidents.. but there's a greater penchant for sympathy for one and not the other.
The law applies to both, but the penchant for sympathy varies. That's why it matters to some (or most) people to specify that he blew a 0.0.
And also when this story broke it was widely passed around that he was driving drunk. While a DUI is still a DUI, for the sake of accuracy, he wasn't drunk, it was on medication.