WOW! I need to collect my thoughts so I can really let loose.
No need to let loose. You pay for a lift ticket or season pass to ski on the mountain. The terrain park is open and you are more than welcome to use any snow feature in it without paying a premium.
The airbag set-up is a feature that isn't traditionally offered at resorts, a feature that most people aren't interested in using (and thus would object to paying for if it were lumped into their pass or ticket cost), and required a capital investment. Mammoth has every right to recoup the investment they made from people who plan to use the service.
I do not plan to use the service and would object to paying for something I didn't plan to use.
You might argue "well Mammoth should just use the money they are already charging us and let everyone use this for free" but that would result in diminished services elsewhere in the resort. You see, as far as I know, Mammoth Mountain is not a charity; they are a business. Their purpose is to please their shareholders while providing an enjoyable on-hill experience. You are already a pass holder, so it seems like you enjoy the hill plenty fine without an airbag to practice stomping tricks on.
Ski lessons, rental and demo equipment, and on-hill dining are not included in the cost of your season pass. They are additional services that you can purchase above and beyond the season pass you have already paid for. What makes the Stomping Ground any different?
If you really want to use the airbag, fork out some bucks and enjoy it.
TL;DR: I don't want to pay for something I won't use. Mammoth should charge those who do.