Maybe I am not the right person to respond to your question as I do not have personal volunteering experience.
But I live in this beautiful part of the world and have on many occasion paid some of these 'volunteer' establishments a visit. Yes, most, if not all of the establishments with volunteer programmes are commercial enterprises that also make money out of us, paying visitors.
Recently I have, on behalf an interested member of a list similar to this one, investigated a few of these establishments, basically to see what they do, and what they offer. It was a very interesting exercise and I have come to the conclusion that I must start some volunteering project. It is a nice, easy way to make money.
First, they get you, the 'volunteer' to pay them to become a labourer. Yes, a labourer. To them it is money in the bank, the work gets done, and they do not pay for the labour.
Second, you will be depriving some local of income. I can assure you that unemployment is a serious problem here, and by employing one local to do the work the volunteers do, can feed a whole family. (Unemployment is also a very big contributor to our notorious crime statistics.)
Thirdly, you are not getting away cheap. Cost of volunteering is high. For the same amount of money you can live in a 3star establishment, with meals, and visit the same big cats as often as you like, get as close to them, and not do the dirty work that you will be required to do as a lobourer.
I have nothing against volunteering projects where underprivileged people benefit. But working as a "volunteer" for some commercial enterprise does not seem right. At least not to me.