It does seem a massively harsh law.

On the situation here though, surely the bar have the legal obligation to check the age of the customers, and the patrons have a legal obligation not to buy underage. The bar were getting punished for the first one, which the patrons can do nothing about. It's not their problem if the bar doesn't fulfill its obligations, it's their problem if they don't fulfill theirs.

You shouldn't be able to sue someone for giving the opportunity to commit an offence, which you then commit, whether or not they're committing one or not.