If you were forced to land somewhere really expensive and it costs say 300 quid, so what? You've spent some thousands learning to fly and the flight you were on right then is likely costing you more than a hundred, so you can probably afford it. This isn't happening every week, maybe once every 100 hours say and even if you only paid 50 quid an hour for your flying 300 is only 6%. I realise this is a generalisation but surely it is generally reasonable?
Then there is the problem that the cost of the landing fee would have an effect upon the decision making process of the emergency landing. Well if that is the only reason to make emergency landings free then I think it is a nonsense - people should learn to make sensible decisions and be responsible for their actions rather than just taking away this factor.
Some are free, they provide a great service and that's great, good for them. Perhaps it helps build their business because they get a good reputation and people are more inclined to patronise them. But I don't see why those that choose to charge should be made to change their policy. Like I said (somewhat unpopularly) in another thread, if you don't like their policy don't go there. Oh, you've got an engine failure in their overhead and you can't make it anywhere else? Pay the money and be happy someone chose to run an airfield right where you needed it most!