Lee, to make it look nice to sell. Motor oil doesn't go rancid if you leave the pan on the shelf for a while, and food grade mineral oil is expensive. If you're just going to hang it as a decoration and never cook with it, then there's no harm.
I said the same thing about WD-40 after I bought a pan coated in the evil goo. >:(
It just goes to show, if you don't know where the pan has been and what it was used for, strip it and re-season it yourself. I find the peace of mind makes me much less prone to tummy aches, too. ;)