Volunteering is different to working for free
From showbiz, volunteering is fine if it's for a community, not for profit or purely amateur organization.
Working for free for a profit based company that pays other employees, or formerly paid other employees until you showed up and undercut them is scabbery of the highest order.
Dunno if this translates to aviation but that was the unwritten / MEAA rule in the entertainment industry, which (as with aviation) frequently got broken by short sighted, self serving desperadoes who did other people out of paid work.