That is right. A landowner would have to convince the court that s/he's incurred a financial loss or was suffering intolerable nuisance to pursue a CIVIL case against the trespassers concerned in order for the court to order appropriate compensation or injunct the particular trespasser(s) not to do it again. Merely parking inoffensively by the roadside does not meet those criteria and it isn't actionable.
In the case of someone doing the parking in a more organised way with intent to bring about a financial loss or to create intolerable suffering for the landowner by design then the police could involve themselves because it has become CRIMINAL.
Much as it may please a minority group within caving to frustrate access for everyone else, which may include intimidating them concerning parking rights, if you do not set out to be an intolerable nuisance yourself and allow yourself to be the cause genuine financial loss to others then you have nothing to worry about in parking considerately by the roadside for a benign countryside pursuit like walking, running, watching birds, having a picnic, landscape photography, flying a kite and all the rest including caving.