Because no matter how pissed he was, there is no verifiable evidence he was over the drink drive limit. Someone can drink a bottle of whisky and be under the limit and others be over after a glass of wine. The limit is set by law based on the reading in the breath/blood and there is no evidence of any alcohol in either. Unlike North Korea, criminal convictions can only be supported by worthwhile evidence.
The limitations of this particular law, is why refusing to give a specimen is a separate offence but by the time he’d been pulled there was no point in requiring him to give one.