All employment contracts will have drug and alcohol clauses in them, however we're talking about people's free will and free time here. The club can't legally tell players they can't drink during the season.
There's the moral standpoint that as professional footballers they shouldn't drink given the vast sums they earn, however it's been proved time and time again that footballers like a drink so it's not like it's going to change.
If I'm a player at a football club earning a relatively modest £150k a year, can I drink? What if I earn £300k a year or £1m a year? At which point do you start getting offended by a player drinking?