I suppose that it is all about the market economy though. "Fair trade" is really just protectionism under another name. That is not to say that it doesn't necessarily perform a useful role, but it does have a destabilising effect on the market and is in effect just a subsidy for foreign farmers.
You say that the supermarkets gain from "other people's misfortune", but I think that is unfair. It is a market place. Without the supermarkets most of those people wouldn't have a livelihood at all, and if they aren't happy with the deal they get, they should sell to a different chain.
As for the top up on 'fair trade' products, I imagine that it is pretty much in line with the mark up they put on all of their products, and is largely dictated by what they can get away with.
Incidently, I am sure that I read somewhere that the British pay on average 40% more than world food prices largely due to the EU trade protectionism and the CAP that seriously distort the natural market prices.