I like how everyone is saying that Egg is issuing "false" reasons for dropping customers.
Egg are a business. Like every other business, they are free to evaulate risks to their business in any way that they want. If I was a shopkeeper and decided that people who spend less than £10 in my shop were a risk to my business, that's my perogative. Just because you don't agree with it, doesn't mean that I'm not allowed to make that evaluation and subsequently act on it.
It's the same here. Morally, I suppose it'd be nice if all businesses told the absolute truth about why they do certain things, but in reality that's never going to happen for many reasons. Egg could genuinely believe that people who have good credit ratings are a risk to their business. You might disagree, but that doesn't mean that Egg are wrong to do that.
As Dolph said: they're a business, they can do business with whoever they want, and if they want to stop dealing with someone, they're not obliged to provide an honest, or indeed any reason for doing so. If you don't like the manner in which they deal with people, you don't have to deal with them. If enough people did that, Egg would soon see that their conduct has caused a measurable loss in business which, to them, will either be acceptable, or not.