True. Maybe he didn't think it through properly and just acted quickly to rid him of any association with Voldemort. And as he considered Volemort dead (he wasn't told about the horcruxes), he didn't think he would ever have to worry about the object again. Still can't think why LV wouldn't include not giving the horcruxes away in his Vow tho???
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Maybe ol' Voldy didn't think that his most loyal followers would have the audacity to give away the thing they are held accountable for hiding.
Or maybe Voldemort didn't want to lead on to what the objects actually were so he didn't give them enough information to figure it out...he just said, "Hide this or you will die a painful death."