The reason for this, is that the American military is admittedly "stretched thin" in Iraq and Afghanistan.
So let's go review what happened a little over a millenia and a half ago, when the cost of maintaining its vast empire utterly taxed Rome's martial capability. With not nearly enough proper imperial denizens to draw upon, the Roman government eventually acquiesced to enlisting barbarians from Germania and Gaul to bolster its forces. In many cases, the foreign soldiers were offered Roman citizenship as part of a package of incentives (which could also include some lucrative latifundia).
Fifteen hundred years later, and another great world-spanning power... is now doing much the same.
Don't anybody give me any harsh looks now. I'm just the historian here.