But it isnt their garden!!
If it was then it would be fenced off and private. Its a field with no designated "your bit, my bit" going on, so why on earth would the OP think for one second that a bit of grass that looks like all the other bits of grass, is off limits?
Choosing to spend stupid amounts of money on a temporary dwelling with service charges on top is your choice, it doesnt actually buy you rights to any land! Its said often on here "You dont own the road...." well there you are!
So do you also freely walk across people's front gardens in new-build estates where they don't allow (or people simply choose not to/can't afford) any fences, walls or other physical boundaries? What about people's drives when they don't have gates at the entrance?
The grass will be cut regularly in one go by the site maintenance person (and charged for at a hefty rate, as you say), so they'd have an awful job doing that if each pitch had fences. Also, most site owners demand you replace your caravan every 10 years or so/when they deem it's starting to look tatty, so how would the low-loader lorries be able to load and unload vans if they all had fences in the way? Not to mention regular delivery of gas bottles.
I agree with you that it seems a foolish amount of money - upfront and ongoing - to spend on a temporary home, but I'm not snobby enough to think I have the right to ignore their privacy (that they've paid a fortune for), just because they've made that financial decision. I personally think it's absolute madness to spend tens of thousands on a brand new car, but many people think differently; and it's their money to spend however they choose, not mine.
The bits of grass between caravans don't usually look exactly the same as the common areas - there are usually roads/tracks/clearly marked pathways and signs up to show which are the public areas to drive slowly or walk through.
How do you actually know how much of the pitch is earmarked on the lease/contract before deciding that you can just go ahead and ramble across the green bits?
Sounds quite arrogant to me to slap down people and ignore their clear privacy, just because you deem them less worthy than people who choose/can afford to live in 'proper' brick-built dwellings. Is that the next stage down in the rights and dignity pecking order from people who do live in normal houses 'but are only renters so we homeowners get priority'?