If it was only the one extra pint then sure grand, probably worth pushing the envelope for, but when it goes on all night and he doesn't show up the next day? That's just unprofessional. If he got away with it other players would rightfully start asking why they don't get to wilfully break rules and protocol as well. It undermines the coach's authority.
There is some wriggle room for exceptional players, but Russell backed Townsend into a corner on this one, if he'd let him get away with this any claim of authority over the squad is gone.
There's a reason Barca haven't won the champions league in years, that's a squad full of talented players with the life sucked out of them by a ludicrous overeliance on the cult of one man (and let's face it even in rugby terms Finn Russell is about a gazillion echelons below Messi for talent, not to mention that as far as I know it Messi has never gone out on the **** and not showed up for training the next day). Barcelona is not something any sane team wants to emulate.