A possible explanation for this could be that some people might believe Obama when he says his plan doesn't intend to help illegals, but nevertheless they just don't trust the government to get it right over the long haul.
Also, the poll shows that a substantial minority subscribe to some manner of Deatherism: " If Obama's plan became law, do you think senior citizens or seriously-ill patients would die because government panels would prevent them from getting the medical treatment they needed?" Here we have 41% saying that this would happen, to 57% saying it wouldn't.
Also, people widely expect a full government takeover of health care happening somewhere down the line: "If Obama's plan became law, do you think the federal government would or would not eventually take over all aspects of the country's health care system?" Here we have 55% that it would happen, to 42% that it would not happen.