Because they can! When there was a shortage of waitresses and JCs in some girls camps they dropped their fees – which worked!
Also, there are expenses involved in every additional person on camp grounds. Food, trips, insurance, etc. For the boys there are rebbeim that are well paid (to attract quality) even for the staff.
The “pay” from the camp is a token, as are the tips. But as tips are required (in most camps) the staff DO depend on them as their summer earnings.
As a former staff member and soon to be staff parent I am totally ok with this.