The violet veil does two things, depending on what tries to cross it:
- When a creature tries to cross it, the veil plane shifts that creature away.
- When an object or effect tries to cross it, the veil disintegrates that object or effect.
A creature using a reach weapon to attack through the veil is neither
plane shifted nor
disintegrated: the veil explicitly exempts such a creature. However, the weapon itself still has to pass through the veil. When the weapon touches the veil, the veil disintegrates the weapon (not the creature holding the weapon). This is sort of like how you can cast
dispel magic against a creature to dispel their buffs, or against a magic item to suppress its enchantments for 1d4 rounds.
Your rules compendium quote describes how worn objects suffer "collateral damage" when the creature wearing them is subjected to a magical attack. But the veil's
disintegrate is not against the creature holding the weapon - it's against the weapon itself.
Disintegrate describes how it can be used against objects: "When used against an object, the ray simply disintegrates as much as one 10-foot cube of nonliving matter." Your quote from the violet veil's text exempts a creature using a reach weapon, because, unlike a creature attacking with a non-reach weapon or natural attack, the creature doesn't have to go through the veil to hit someone inside. The reach weapon goes through the veil instead, and when it touches the veil it is disintegrated.