This seems exploitative. don't the venues have professional stage hands?
not usually, at least in SW Florida. Bands set up things themselves and run their own sound. Band makes $600... they'll often pay a dude 40 or 50 and save them the time of setup. Most are lazy and would prefer to just walk in and play, or get drunk while they play, and hook up right after, etc. For the amount of time involved, if you live close and would be hanging out at these clubs anyway, if you are creative enough to set up 2-4 clubs a night, you can make decent $ and have a fun night doing it.
VERY top bars will have a sound or light man, but most bands run their own sound from the stage and have the auto lights that smell the kick drum and move in sequence with the song.
If you invest in a PA system (2 k12s, 2-3 K8s, a sub), you can run your own PA system and charge bands 150 or 200 a night. A few guys do that. I might do it when I get old, once I'm too fat and bald to play lol. They set up, they read or work on computer or drink and relax for a few hours on their ass, and they really control their own environment. I did it one night, it was fun but preferred the stage to it.