"mandatory" where does the mandatory come from?
On nights I went to Bar Fama with friends, and if the place was too empty for us to stay, we left after five or ten minutes and paid only the entrance fee of R$10.
We were informed that if the place was busy with customers and garotos, and if we stayed in the facility to partially watch a show or "cruise" for more than ten minutes, we would be charged R$20 reais consumption charge on top of the R$10 reais entrance fee.
In reality, it is not necessary to enter the facility to see if it is too dead to be interesting. Just pull back the curtain near the entrance and look inside. If it is dead, you leave. Disappointing, but quite a simple maneuver.