Volunteer management is something I know a little bit about since my wife used to run a Volunteer centre and now spends her life running about the Government putting volunteering onto the political map. So by a process of sublimation I've learnt a bit about how it works.
Volunteers cannot be "told" to do anything unlike a paid member of staff, basically you generally have to suit the role to the individual not the other way around. You have to have a code of conduct and the volunteers have no recourse in law for employment rights
The definition of a volunteers is they are working for the benefit of a third party, not for your own benefit. There is an incredibly fine line between a volunteers role and job replacement.
How is working for free benefitting society in the case of an FI?
One of the big problems with how FI's are paid, is that just because you aren't in the air you are still working. Answering 'phones, dealing with potential customers etc.
Unless you are an FI you don't really have any idea of how hard the job can actually be.
BSAC instructors aren't often full time divers, but if you want an example, how about you go and tell the divers in Aberdeen who are working and earning other people money that they should be happy to spend weeks in a diving bell and do it for free.
The fact is that we are losing FI's because of pay. Life has been getting better pay wise thanks to a fairly dire shortage of FI's, so in this case the laws of supply and demand are now skewed towards the FI and especially the experienced FI who can teach more than just the basics.
I know of a certain school who have been trying to attract an experienced CPL FI, but even for a salary of over £30k, there haven't exactly been many takers.