first: it's RFK that they're talking about, not JFK.
-Huckabee's comment was a badly fumbled joke where he was desperately searching for a setup for his punchline about Obama tripping over a chair. It was a stupid thing to say but the context was clearly understood as a failed attempt at humor. He should suffer some sort of fallout for it if he is in contention for the VP slot, if only as an example of the 'foot in mouth' liability he represents.
-Clinton's comment is different as it wasn't a joke and it can be interpreted as an insight as to what she's hoping for in the nomination race. As almost all the numbers are against her, the only hope she has of getting the nomination is if something disastrous happens to Obama. While polite folks might offer the possibility of a legal scandal, medical issue, drug history, or some "cultural values" skeleton in the closet, she's the only one to tread into the assassination scenario. She's touched on it a few times before without specifically mentioning the "killing" of RFK.
-She's getting lambasted by the media because despite her assertions that there are people who have threatened her campaign because they won't tolerate a female president, Obama received actual death threats the day after declaring his candidacy. While most people understand that she wasn't calling for someone to go all John Hinckley Jr. on him, there are always going to be disturbed individuals who can be set off by these sorts of comments. The guy who stabbed Monica Seles during a match was doing it because he wanted Steffi Graf to be the WTA #1. Someone running for the highest office in the land should be savvy enough to know when not to mention RFK's assassination during a campaign. As in NEVER.
-She's being made fun of because if her only chance of being president is a bolt of lightning coming down and striking Obama, then the validity of her candidacy is seriously questionable. It would be like a CEO cooking the books, getting in deep with loan sharks, perjuring himself during the Grand Jury investigation into his company's problems and relying on a lottery ticket to get enough money to bribe everyone to make it go away. Yeah it could happen, but you wouldn't want to rely on it.