I hope this is the right forum. want to know why volunteer groups charge money to apply for a volunteer job.?

I have a lot of experience traveling to 3rd world countries teaching, digging wells, and building or repairing buildings. However, have not done so in over 10yrs. I found a dozen that are mini peace corps groups, meaning volunteering in 3rd world countries fo 3-6 weeks, mostly in Africa. my first 2 applications went OK, but the last 5 wanted $20-$40 fee to submit application. I have a degree, and experience in many areas needed for these groups. I plan to live in a tent, and eat not the best food, and certainly, not what I am uses to, for 3-6 weeks. So it bothers me they want me to [pay to submit an application. ANyone know why.