They still do checks on arrival for train 9057 from Marne-la-Vallée–Chessy so that's not necessarily a reason.
No doubt the risks have been assessed between frequent regular services from major cities, and those of a once daily special service from a children's resort, mostly booked as part of a return package, and arrangements made accordingly. The world is not black and white, and just because something works in one (unusual) situation does not mean it is appropriate in another.