Someone with a pre-existing condition should be denied insurance. However they, like anyone else with a condition, should be aware that their plan may cost a little more or less. It is better for them to be charged differently than to be entirely denied access. They have to consider the risk to their own company when signing people.
I don't fault health insurance companies for wanting to cover themselves and charge higher for pre-existing conditions, where my problem comes in is when people cannot get insurance because of pre-existing conditions they have. Everybody that wants coverage should be able to find a way to obtain it, problems or not.
I think there needs to be a trade off between penalizing people who already have a condition (especially if their behavior may have exacerbated it) and ensuring coverage for all with reasonable rates. I also do not believe a pre-existing condition should be used as a denial of coverage. Rather, perhaps those folks should have to pay 10 or 15 percent more. Nothing exorbitant, but something to acknowledge that there is a difference.
Of course health care companies should be allowed to accept applicants with pre-existing conditions IF the applicant is willing to pay enough to be covered. However, that cost, quite reasonably, will be likely be extremely prohibitive. Generally, when this topic is discussed, it's done so with the notion that companies should be forced to cover everyone at the same price, which is not feasible.