Each value for the DisabledSlots tag is supposed to disable one function of one slot, but the value 1 disables the removal of all slots, the replacement of all slots, and the placement on the main hand slot.

I believe this is not supposed to happen, as now it is impossible to make an ArmorStand that have both of these properties:
1. the player can remove and replace items on its main hand slot
2. the player cannot place items on its main hand slot

Whilst for all the other slots it is possible.