They will hire someone for six month then at the end of that time another person will come in and take if for six months. Or as is becoming more common people are put into 1-2 year appointments that are supposed to only be used to fill in for situations like when a permanent employee is on military deployment, or extended sick leave; or a limited term project . Instead this illegal to avoid paying benefits to Park Rangers is used for people doing everyday on going operations.
Because of their love for and dedication to the resources of our national parks many rangers put up with this abuse and the National Park Service know they will just sit there and take it for that reason.
This violation of federal employment regulations, that is rampant in the NPS, is the same kind of practice that causes people to castigate major retail chains like wall mart. So next time you visit a federal park and you feel uplifted and well served by a friendly helpful ranger know that there is a good chance that this is what they are suffering.