Permanents that have the indestructible ability cannot be destroyed by effects that would destroy them or, if the permanent is a creature, by receiving lethal damage. However that does not make indestructible objects illegal targets. For example, a Darksteel Axe can stil be targeted by Shatter. When Shatter resolves, nothing will happen, but that does not mean that the Darksteel Axe was an illegal target. Compare this to a situation where an Darksteel Juggernaut is targeted by Divine Offering. In this instance, the Divine Offering would not destroy the Juggernaut, but the controller of the Divine Offering will gain five life.