Why can't you throw a bow? Are they covered in tanglefoot goo? If you take the rule to its english language end, then every single weapon is a weapon that can be thrown. It isn't necessarily a weapon that can be thrown at someone to deal damage, but you can most certainly throw it.
That said, I'd don't see a game balance reason to disallow Returing on a ongsword, but there are probably other ranged weapon enhancements that wouldn't be proper (seeking springs to mind), unless it was ruled that it only worked when the weapon was thrown.