Yeah, interesting point.
My thoughts on that is that Sparda did not put the sword away 'in' the Demon World proper, it's located in the 'Unsacred Hellgate' which seems to be a kind of dimension all its own, between the human world and the Underworld. The place where Arkham takes the sword for himself is that dimension, and the area where all of the Escher stairs and the area where Dante and Vergil have their final battle are all parts of it, too - that wasn't 'Hell' itself (even though when the amulets were joined they allowed access to the part of Hell where Mundus was). The Underworld was below that somewhere, and I guess we can assume that Sparda placed some form of separating seal on that place where the sword was, or created that dimension, so that any demon couldn't just waltz in and take the Force Edge, nor could any human from the human world. It would make more sense if he hid he sword in the human world rather than in the demon world! But it would make even more sense if he hid it in some limbo-world between the two where it would be hidden from both humans and demons.