Sounds like you live in a pretty unusual place, where there is NOWHERE to walk or play within walking distance but there is a trampoline park!
I've lived in lots of places - inner city, suburbs, big housing estates, small/medium towns, and have never been more than 5 minutes away from either a play park or some sort of green area(usually far less than that, about 2 minutes). I'm thinking about where my parents, siblings, friends, live, and they all have somewhere green they could walk to within 5 mins or less, but you have absolutely nothing within 15 minutes?
You understand the trampoline park doesn't consider its customer base people who are within walking distance, but is covering a much wider area? It's mad you expect a BUSINESS to not meet costs, or even lose money, to subsidise someone who CHOSE to live where they live because you can't afford to run a second car and "can't face" either using public transport or staying at home.
To be blunt, why on earth should they care?
And tbh £15 probably isn't making them a huge amount of profit and is close to 'what it costs them to run'- with new minimum wage each staff member will be costing them at least that per hour even when they are on minimum wage, by the time you include pension, holiday and sickness pay, etc.
Then there's the rental costs (massive), heating (going up every year), insurance (going to be very high somewhere like a trampoline park!), purchase and maintenance of the trampolines, not to mention all the normal business costs like advertising, cleaning, accounts, HR, paying for booking systems, admin, water bills, electric, council business rates....
You sound both naïve and pretty entitled. If you're that annoyed why don't you be pro-active and ask them if they'd consider reduced rates for local people or monthly subscriptions for people who come often rather than paying per session?