That's kind of my point, I don't think anyone goes and presents the breath test on its own. They're like "I was trained for X months to identify drunk drivers and use this equipment, and I've stopped X hundred drunk drivers, and I've been at parties where people are drinking, so I know what levels of drunkenness are like, and the machine was calibrated and they failed the walking and eye tracking tests and their pants fell down and they pissed themselves and they had a blood test of double the limit..."

The breath test is, I think (I am not a lawyer) insurance that there was reasonable suspicion of the person being drunk, not final proof of anything.