They way I was taught, is that there were two types of Continuous Trap Cards- Those that are actually continuous, and those that trigger when something happens/you activate it/etc. Basically, actual continuous effects can be chained and have their effects applied immediantly when flipped ("Royal Decree"), while trigger-kindof Continuouses could not do so ("Royal Oppression").
Those kind are so because they can be flipped and chained, but not applied, because you can activate their effect at a certain time, and it can be chained because it actually uses the chain. In a relative example: "Wave-Motion Cannon" can be activated, and sit on the field for turns and turns. When you decide to send it to the graveyard to activate its effect, it's put onto the chain, and cards like "Pikeru's Circle of Enchantment" can be chained to decrease the effect damage to 0.