A further thought I had was that some money grabbing owner of a busy club or bar could argue that while his bar staff were serving you a free drink, they could be serving a customer who is going to spend money.
As I'm well past the age to go clubbing, and I prefer a pint of real ale in my quiet local, I wasn't aware that things got so hot in clubs nowadays, although having watched Ibiza Uncovered and seen the inside of Manumission etc I suppose you would need a drink. I suppose they also sell bottles of water at grossly inflated prices?
Strongly resisting the temptation to say "In my day............."