It's a liability. People don't take responsibility for their actions anymore. People are suing for anything and everything. If somebody builds a jump on a trail that isn't marked and somebody some gaper comes and eats shit on it, things can get ugly.
There's a reason places have terrain parks on designated trails with the smart style signs like this at the top of the park.
Mountains spend a shit ton of money on insurance just to operate, no need to add any extra problems. If you can tell them to stop without being a dick and explain the situation sometimes it's what you gotta do. Besides, if there is a marked terrain park at OP's mountain they should just hit the features in there.
I used to build jumps at my home mountain all the time when I was a kid. That said they didn't have a terrain park of any kid. Also life seemed to me more lax back then.