In Indiana, no one gets inside the park without paying unless they have a season pass.
Camping areas are always restricted to ONLY paying camper and are stopped at the gate house before entering the actual campground area. Sometimes, they will let someone pass through if they want to see the campground for future camp site selections (which we've done ourselves). But mostly, if you are not a paid camper, you are not permitted in the camping area.
Personally, I like this rule.
Even in those State Recreation areas and State Parks that have horsemen campgrounds, if you are not a paid camper in the horse campground area, you don't get in, even if you are a paid camper in the regular campground in the same park.
Now if you are walking a trail or riding a bicycle, almost anyone can get past the campground entrance. But if in a car, you're not getting in unless you have a reason to be there.
Personally, I like that!