Sydney's facination with laundry seems to be related to the laundry basket itself. She only messes with clothing in the laundry basket (clean or dirty). Something about climbing into the basket and pawing through all of those clothes must be fun for her. But when laundry is done and there is a drying rack full of undies in my bathroom well within her reach...she doesn't bat an eye at it.
Socks however are a thing of their own and those are fun to chew on even if they still have feet in them.