Blame deflection definitely makes sense as a reason - in which case I am surprised we don't say this more in the UK as we love avoiding blame. And the fact that this has become part of a training course could explain why it has spread across the US. By the way I've just realised that the quote from You, Me and Dupree should have been "I need you to do _me_ a solid", which I think you'll agree is even more annoying because of its emphasis that a banal or tedious act is being requested as a personal favour.