I don't think it's trespassing for the players to be at Kijkduin, just problematic to have people there in those numbers. The players may be making individually-acceptable choices, and it's just the collective behavior that's a problem. It's easy to say that people should have the judgement not to do some individually-harmful thing. It's harder to argue that individuals should have the judgement not to partake in a harmful collective behavior (humans just aren't generally good at considering things like that).

Niantic's algorithms are sending people around in a pattern that is causing damage. Since they've been made aware of that, and it's difficult to blame the individual players, it's reasonable to ask the company itself to make a change. Niantic's the only one in a position to solve the problem quickly and cheaply.