Poor analogy. You’re being asked to come to a gig and play for the benefit of others. Many such events may well feature paid bar staff, some of whom may just have started doing such work. They still deserve a wage.
There’s a difference between unpaid and very poorly paid. Also a difference between unpaid overtime (implicit in some employment contracts) and volunteering.
Because they know better, apparently. Pressure not to complain. Go look at twitter; many small acts have a hard time having to play for nothing but feeling like they can’t do anything about it.
And so you perpetuate the problem!
Your practicing was in the studio/bedroom/garage surely. If you’re being asked to perform your skills for the entertainment of others, you ought to be paid.
The poster was specifically referring to a hobby at the weekend. You’re also not depriving professional footballers of income by playing for free at your local park.