PPA works hard to recruit member parks as well. The benefit to parks is in enticing travelers to stay longer than they honor the PPA discount for, thus reducing the overall discounted rate. In business lingo, its called a lost leader.
For the traveler without a set plan savings can be made for short stays when the park has availibility.
Think about parks who offer PPA discount for more than short term, or who restrict discounted sites to a section. Oft times, you get what you pay for, or cannot fit into spefic sites they offer at 1/2 price.
Its a gimmick for both parties. If it was a very good one, you would see more members on both sides. We choose not to be one.
Max