Name ANY other industry where people work for free for companies that are making income off the activity you are performing for free for them ? Do people work for free at the local servo to get cash register experience to work at Coles?
Certainly not with jobs requiring less formalised training such as those you list, but for example legal professionals often work for free during an internship to build experience and contacts. There are other industries where this is happening with what should be seen as detrimental effects (game design and film/television for example), but certainly in the legal profession it is seen as a positive thing.
I'm not saying this translates to aviation, and I agree people deserve a fair day's pay for a fair day's (or night's) work.