I think it has to do with the fact that for US carriers fuel is charged as part of the ticket price (and therefore included in the redemption) while non-US carriers can include the fuel surcharges separately.

If you think $550 is unreasonable, try booking a redemption on Virgin Atlantic. In mid-2009 when fares were really low, I tried booking JFK-LHR-JFK in economy using Virgin Atlantic points, and the fees and taxes ended up being almost the same price as paying outright for an economy fare.