Logic sometimes escapes the legal process.
The government has held bartenders liable for diagnosing who has had too much to drink for a long time now.
A good defense attorney would put the defendant on the stand and ask if the defendant is a medical doctor qualified to quantitatively assess the blood alcohol level of a customer. Ask again, is the defendant trained as a law enforcement officer to perform field sobriety tests on customers sitting at a bar. Ask again, is the defendant provided with calibrated breathalyzer devices or a lab to perform a blood draw on a customer. If the answers are no, then the defendant with simply be guessing about the level of intoxication of a given customer.