That's not how he phrases it --- he actually proposes that Palestinians handle internal security, and a joint U.S./Palestinian force would guard the borders. But the parcels of land involved are so tiny --- the Gaza strip varies from nine miles wide down to four --- that in the presence of serious, determined resistance forces, that would amount to a distinction without a difference. At least initially, the U.S. force would have to be large, and being large, pervasive; there simply isn't physical room for a border zone which could fit a large U.S. force without getting in the faces of the Palestinian citizenry as a whole. Which, in turn, would make claims of Palestinian sovereignty ring as hollow to the Palestinians themselves as the autonomy which they now supposedly enjoy.
And even if you believe the territory is large enough to fit separate zones of jurisdiction for a border force and internal police, you still have to find internal police who you can trust not to harbor terrorists. Between the Karine A affair and the various affiliates of Fatah who keep coming out of the woodwork, the current Palestinian Authority has some problems there.
And of course, the forces in the Arab world which are trying to play the "Crusader" card in fomenting war with the U.S. would get to cut out the middleman --- an explicit part of Friedman's plan is for the United States to occupy Jerusalem, including the holy sites.
So, that's the proposal of "the most important opinion journalist in America", according to Slate. (To call him a columnist would be insufficiently grand).
Here's a suggestion from one of the Internet's less important whiners: if you're going to propose an externally imposed force in the territories as a guarantor of Israeli security, perhaps you might want to find one that's a little more culturally appropriate. Consider Jordan.
Of course, part of the deal would have to be that the Israelis would have to trust the Jordanians to restrain Palestinian terrorists. Which means that they'd have to trust King Abdullah to treat terrorists threatening them the same as his father did when Arafat's crowd became troublesome to him --- to make a long story short, he kicked them out.