Is it immoral for a health insurance company to refuse to cover a person with a
Timely! I'm inclined to tinker a bit with the question.
First, I think it's a scandal that in the USA, people with pre-existing conditions often can't get health insurance. Other developed countries have figured this out; it's about time the US caught up.
However, given the way the system works at present, we might get a fix on your issue by asking this. Suppose I'm in a position to set up a not-so-large company that provides health insurance, but I'm not willing to take on the risk of insuring people with certain pre-existing conditions; I'm worried that if I do, the company will be bankrupted and then won't be able to insure anyone. (I'm not saying this is actually the case for most big health insurance companies, but bear with me.) So long as I'm upfront and honest about what I'm offering, it's not clear that I do wrong by offering my more limited product. It's in the nature of private insurance schemes that the companies are in the business of risk management. Companies that don't make appropriate risk calculations in deciding what premiums to charge and whom to insure probably aren't going to last long and arguably aren't responsible businesses.
So far, then, the fact that a company won't offer policies to people with pre-existing conditions doesn't automatically make the company itself immoral. Note the word "automatically" here. I'm not suggesting this hypothetical scenarios is a good fit for typical big insurers. But to repeat what I said at the outset, there's something very wrong with the real-world situation: the US, the world's wealthiest nation, leaves large numbers of its citizens people in this awful situation.
It's worth adding: the very idea that the market is the best way to deal with health insurance is eminently questionable, as this essay by Nobel-Prize winner Paul Krugman points out. Where to place the blame and what we do about it is not easy to sort out, but if health care reform gets bogged down in the sort of mindlessness we've seen of late, then there will be a long list of villains whose names should live in infamy.