Riise has no right foot worth talking about and I agree with the original post that it's an absolute disgrace that some professionals can't use both feet. He's not the only one though, there are plenty of them about. In fact every team in the premiership will have a couple of them. There's nothing you can do to change players like this in my opinion, the only thing you can really do is work with youngsters coming through and practice a lot of ball work with them using both feet.
The fact that some players are just too scared to make a mistake with their weaker foot is a good point. But again, what can you really do about this? As others have said if a player's in a confident mood he'll use his weaker foot more, if not he becomes a one foot wonder. It's just the way it is.