Looks like a bunch of libel to me. How do they get by with it?
Same reason the tabloids can. The courts have basically said that for "public figures" (i.e. people who are famous), the only way libel can be found is if there is a "reckless disregard for the truth" that significantly damage the reputation of the person involved.
The plaintiff would have to proof that the accuser knew the allegations were false, did so anyway out of some personal hate and that the accused suffered damages as a result of the libel.
Most celebrities also know that the cost of fighting such rumors/charges in court only lend credibility to the rumors/charges ("why else would they fight them?" some argue) and broaden the circulation of the rumors/charges more than the original accusation circulated.
Let's say, for example, that it is published that a white southern congressman uses the N-word frequently in public. It's a total lie but if the congressman chooses to sue, then it becomes a "news item" and now the charge gains wider circulation and credibility than if the congressman just ignores the charge or calls it "baseless" if someone asks.
Their attornies and handlers will usually tell them it isn't worth fighting the lie in court and there is always the chance that a jury won't be pursuaded that the publisher printed the lie "with malice aforethought".
Think of all the awful things said about the president. That he wants black people to die, that he knowingly lied about WMDs, etc. because they know Bush can't really fight the charges. Your First Amendment at work, folks.