I notice that people seem to be following the entertainment, basicaly going where ever there is a show, wether it be a Tribute act, band ect.
So to me it seems that most people are looking for entertainment.
I don't think the bars would go through the effort of finding and paying for entertainment if they felt it unnecessary. But unfortunately if they see that it works they will do it as it seems that people enjoy it because without entertainment most of the bars are empty unless they have karaoke or a quiz which also seems to be very popular.
Overall they are not charging to watch the shows yet they are paying for them and i'm sure there not all cheap, they are offering them for free. You actualy have to pay to see a Tribute act in the UK.
The bars don't put entertainment every night so there is always a place for everyone to go, if you don't like Tributes just avoid them when they are on.