At least with breathalyzing, it's binary. You're either drunk or you're not, there's no middle ground or room for interpretation.
In Canada, I think, penalties are doled out based on a few ranges of blood alcohol content.
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.