I realise it is a bit of a shock but it wasn't unexpected. DDL was expected to win since the Golden Globes as was Javier Bardem.
Marion Cotillard was a surprise but the expected winner was Julie Christie, again not American. And although Tilda Swinton was also a surprise, it was believed the award would go to Cate Blanchett, an Aussie.
It's not like the Americans didn't have any notice of it and it won't hurt for their collective egos to take a bit of a knock. Plus there were plenty of US victories in other categories, Director, Picture, both screenplays, animation etc.
And watch, next year there will be four US acting winners!