Why must Class 5.1 oxidising substances be kept away from combustible materials even if the oxidizer itself is not classified as flammable?

1. Oxidizers freeze combustibles on contact, cancelling fire risk
2. Oxidizers can yield oxygen or otherwise intensify a fire in nearby combustibles
3. ADR requires all 5.1 packages to travel only by rail
4. Oxidizers cancel Class 3 labels when they share a pallet