Hello Vuroikoitt24
I don’t know why, but it is effective not the same validity, you have only a 50% reduction with the STP.
Perhaps the train company receives a grant to transport cross-border workers from Vallorcine to Le Châtelard-Frontière but not for tourists. All other tickets, passes and travelcards are available/valid from Martigny to Vallorcine.
Kind Regards
Danhiel