Many years back I used to instruct at a 'club'* (i.e with Presidents, Chairmen, Honourable Secretarys etc). Club rules said that elected committee members couldn't accept money for instruction.
So, after coming in to work to expect a fullish days flying (at I think £7.50 per flight hour!), I found that many of my flights were appropriated by the committee, thus saving the club money. I then had to wait, with no retainer, for any extra work to turn up.
I'm not saying it will happen with non paid non-CPL FIs, but it's a business environment.
*as opposed to a 'flying school'.